Here's some things I've been noticing from the marine side of things:
There is no tl;dr but I did format it so it's not a total wall of text.
Neurospam
It's cancer. It has no counterplay, and little risk. It isn't even blocked by cades, unlike marine gunfire from the DEFENSIVE side. Spitters, Sentinels, Praetorians and Queens can all spam this as much as they like until they get a hit in, and then they're tanky/fast enough they can run through gunfire, grab the downed marine and drag them away to their death. Being able to spam all the while to make sure the downed marine can neither get up to run, nor pull out a gun to fight.
Solution: Just make neurospit actually cost a substantial amount of plasma. Actually this is a problem with ALL combat xeno abilities - everything can be used willy nilly without even caring about how much plasma you have. Neurospit in particular is bad with the way the spitting castes can machinegun fire it. Changing neurospit to a more reliable but substantially less spammable ability would do wonders to eliminate how cancerous it is without making it useless. In essence, make it an ambush/initiator. It may even be a good idea to simply adjust them to work more like a short range boiler/prae spit.
Queen Screech
This is just as bad as neurospit, in part because it gets combined with neurospit - particularly queens spamming neurospit to grab/drag/slash whatever marines they hit, and then using the screech if they ever get in trouble/swarmed. Eliminating ENTIRELY any risk involved whatsoever. A decent queen doing this on the front lines completely destroys marine pushes by itself, and more generally it's a pain in the ass to deal with as it can drop an entire FOB dead in combination with a boiler attack, and makes all cade lines worthless. The only real counters marines have against this are the SADAR or a well-timed CAS. That isn't nearly enough for just how effective and spammable it is.
Solution: Change the screech's area of effect. Don't make it hit everything the queen sees, have it only hit a cone, or a line in the direction the queen is facing. Limit the number of people that can be hit by the queen screech so it takes some skill to use and isn't always "get out of jail whenever I feel like" card. Consider even just getting rid of it entirely.
T2/T3's Too Tanky
Almost across the board, T3 are too tanky. Crushers are the worst offenders, with Praetorians coming in close second. Only the boiler actually feels like something that can be killed without requiring an entire unga-rush of 20 deltas shooting while chasing it down. Not far behind the T3's are the T2's - which get tanky enough in combination with their speed at elder/ancient - that attacks stop being a risk and become more like toying around with marines, and on top of it all is how much health elder/ancient xenos still have despite being in crit, making it even more difficult to actually confirm a kill on the back-and-forth frontlines with xenos quickly dragging those crit back to safety and body-blocking incoming shots that might finish it.
Solution: It's all over the place, there's so many different changes needed. Crushers could do with less health, praetorians and ravagers with less speed. T2 castes in general are too tanky once they reach elder/ancient tiers, and Lurkers in particular are way too fast for their stealth/damage; A good lurker is honest to god better for the xenos than a ravager is.
Too Hard to Defend Cades
It's not just a problem with Queen screeches and massive gobs of boiler gas covering the entire FOB - cades are completely useless once you extend them enough from the main LZ that a marine can't be covering it 24/7. This isn't just a problem with how weak cades are in general to xeno attacks (where marines have to spend significant resources + TIME on cades while xenos just need 10 seconds alone to destroy it), but also with the map design that makes it impossible to hold vital logistical lines because of the dozens of flanking opportunities from vents and side passages, and then on top of THAT is the amount of time it takes for engineers to put up cades in the first place, and the substantial amount of material used up to do so. Two engineers in a squad can barely make one defensible mini-FOB, let alone securing the path the squad takes. Metal cades are garbage in general, and sandbags are a goddamn nightmare to fill up and use in a timely fashion - you can't even prefill and carry sandbags with you.
Solution: One of the biggest problems with cades is that a spitting caste with 10 seconds to spare can annihilate an entire line of cades that took an engineer five+ minutes to grab supplies, run around, and build. Acid is a cheap and easy way to destroy cade lines and marines have ZERO counter to it besides babysitting the cade constantly, which isn't always feasible given our current maps. Give engineers some kind of acid neutralizer so they can stop the acid and repair the damage done, and change the acid from a guaranteed destruction timer to a Damage Over Time effect. In addition, increase the hitpoints of cades to match the now stronger mutator-buffed xenos so the cades aren't such a breeze to smash through, reduce the time it takes engineers to build cades, and massively decrease the time it takes to fill a stack of sandbags.
Defcon Doesn't Matter
Defcon is neat from a lore/atmosphere perspective, but as a gameplay element, it's awful. There's too many items scattered around the map worth too little to bother with. 9/10 rounds never make it past Defcon 4 and marines have zero incentive to gather that intel in the first place when it just places them at greater risk for zero gain while giving the xenos breathing room and better weight of numbers in ongoing fights.
Solution: The simplest solution is to remove the requirement to 'chain' defcon items together and simply allow full points to be collected for each item when you have it, not when you've done the busywork of reading this and that piece of paper and inputting codes. That would make defcon work more intuitively and be a little bit easier to actually get points for. I'd prefer, however, if this entire method were scrapped, and we moved to a capture-point mechanic, with marines having to hold certain areas and gain intel while doing that, rather than simply playing collect-a-thon.
Fire Helps Xenos
Rather than being the bane of aliens everywhere, fire is actually pathetic. There's so many things wrong with it I hardly even know where to begin - rather than fire resistance, the Queen and Ravager inexplicably have fire IMMUNITY. All xenos are still able to heal WHILE ON FIRE! The damage over time that fire does is garbage - only the Pyro's blue fire is actually something to worry about at all, while regular orange or god forbid green fire can almost be outright ignored by xenos. It's so bad right now that xenos will actually walk into fire, SET THEMSELVES ON FIRE, and drag a marine into the fire with them - because it kills a marine faster and better than it does the xeno. And then there's the Incendiary OB, which, while it's the only genuinely effective OB marines have right now - is ONLY effective because most xeno players panic and try to run out of the fire, taking damage from each tile, instead of standing still and resisting to put it out or resting to outheal the damage. Speaking of which - most xeno castes at elder, maybe even mature+ (I've even seen young xenos do it with pheromones) can actually OUTHEAL THE DAMAGE FIRE DOES TO THEM WHILE RESTING WHAT THE FUCK.
Solution: For starters, remove fire immunity from the ravager and queen. Even if it does reduced damage, it should still DO DAMAGE. There aren't ANY marines that are 100% immune to xeno slashes, spits, or boiler gas attacks, so why do xenos get unilateral immunity to fire??? It also should not be possible to heal while on fire, let alone out-healing the damage the fire is doing, and the fire itself needs to do more damage in general to xenos to actually scare them away. A xeno set on fire should be running away to try and have the fire put out by other xenos or die. None of this, oh no 30% of my HP is gone, guess I'll just neurospit this flamer guy and light him on his own fire then drag him off to ensure he dies while I outheal the fucking damage of the fire. Fire needs to hit like a truck. A big flaming firetruck of fire. Xenos who think they can just grab marines and tussle around with them in fire need to be properly punished for such ridiculous shenanigans.
Grenades Are Completely Worthless
This is pretty much the same category as the fire bullshit. Grenades do FUCK ALL to xenos. ONE HEFA can give 5 or 6 marines IB and broken bones, making them slow, paincritting them to the ground, or disabling their ability to even shoot their gun, nevermind just outright killing the marines. Meanwhile your typical young drone with no mutators can sit its ass directly on a HEFA and live. Fuck that noise. It should DIE. The grenades are so fucking telegraphed already you have to be blind, deaf, and dumb to run into one as a xeno. You DESERVE to die to them. At the moment your typical T2 can sit within 3-4 tiles of detonating HEFA nades and tank a half dozen or more of them. That's like 30 fucking seconds of tossing grenades at a xeno to kill it while hoping none of those bounce back at you, or god forbid you get fucking neurostunned with the primed nade in your hand; it's fucking stupid! HEDP is even worse, doing even less damage while failing to stun xenos for any length of time that would allow anyone to actually move up and finish the xeno off. And that's not even getting into the massive issues with throwing grenades in the first place - just one person running in front of you or playing help-intent games and that's it, you've fucked the entire fob and yourself, and your friends.
Solution: It's not the damage that needs increasing. Grenades already do a fuckton of damage to marines. It's the damage TO XENOS that needs fixing. Xenos should not be brushing off grenades like they're nothing. Grenades should not be so worthless that all the experienced players literally disposal every grenade box they see to prevent noobs from friendly firing the entire FoB with idiotgrenades. Grenades are so fucking bad right now I can barely put it into words. The only thing they're good for is fucking suiciding yourself and friendly firing your friends. Increase the damage to xenos, and let marines toss grenades through other marines.