Synthetics are very strong against Xenos if you simply equip them with a fireaxe. Not only do you have to slash them far more than you do a marine, they don't suffer any degradation of their combat abilities due to all the damage they've sustained. Even when you kill them, you only need to recover the corpse in order for an engineer to rebuild them in their entirety and go in for second round. They're also immune to several Xeno abilities, most notably pounce. At their best, they can slaughter an entire hive if that hive can't focus enough DPS on them to completely eliminate them. At their worst, they're a painful damage sponge that can advance well forward of the front lines, tanking slashes while IFF rains over their head.