I don't accept that it's just that xeno players are bad. That argument has been so overused at this point that I can't even take it seriously. There are always new players and varying skill levels on both sides, and there are certainly no shortage of new marine players right now, or people trying out new roles. Yes, somehow all these terrible bald medics, incompetent engineers and PVTs friendly-firing everywhere aren't impacting the win rate.
An argument that I think is far more legitimate is that because xenos have lower numbers and greater consequences for death, low-skilled players are more of a burden to the xeno team. But that's not something that can realistically be changed. The game will always need new players, and that means new players playing xenomorph and possibly committing the awful sin of being bad. That's not a problem for the game, it helps those new players learn and gives good experiences to the marines who kill them. If the balance hasn't factored in new players existing or their disproportionate effect on the xeno team due to smaller numbers, then that's the actual balance problem. It's not the individual players.
In terms of actual issues with the state of balance right now, I think it's actually quite complex and has a lot of components that combine together. I personally feel that shotguns are currently too versatile (not necessarily too powerful, but too effective in too broad a range of situations). Shotguns determine every engagement to the point where the entire mentality of the playerbase is built around them, and they leave tanking castes (defenders and crushers) with no real role in the current shotgun-dominated meta, other than maybe to sit in front of boilers. You can't tank buckshot, ever, and since buckshot is ubiquitous these castes massively decrease in utility.
Speaking of boilers, another problem right now is extreme reliance on a few roles or castes which basically function as keystones for the entire team. Boilers are the really obvious example, they are absolutely determinate in a way no other T3 caste is, which means they need to stay alive and they need to be good. You can't siege without them. You can't defend without them. You basically need them to carry the entire xeno team, and that puts a huge ammount of pressure on the few players who roll boiler. It doesn't help that boilers are also the most vulnerable caste when it comes to the aforementioned Fatty/mortar meta, so they can die very easily to very minor mistakes. All in all, you have a caste which a lot of experienced players don't find particularly enjoyable (sure, there are exceptions, but robustos generally want to be on the front lines), but which also has to carry the team.
Finally, I don't think it's determinate, but I think the changes to capturing have really hurt. Depending on time of day, marines have a constant influx of new players over the course of a round, and while the loss of specialised roles may really hurt that mass of new join squad marines can keep marines going even in a bad situation or after an initial loss. Xenos, on the other hand, are very unlikely to ever recover from serious losses other than having a boiler sit in a chokepoint (again, see boilers having to carry the team). Capturing is difficult and becomes more difficult the more marines outnumber xenos, and the rewards are just not good enough to even keep up with attrition most of the time.