Winrate is a shitty metric to try and understand CM. It can be indicative of problems, but I dont think it effectively describes a design goal.

To me, CM is less a team deathmatch game and more of an exercise in collaborative storytelling. Similar to a tabletop roleplaying game such as Dungeons and Dragons. Antagonists like the Xenomorphs only exist to drive the plot of the story forward. They are a tool, much like a car, that serves to bring the story to its intended completion. Its better to think of them more as an extension of the environment than as "the other team".

So xenos winrate don't describe bias so much as the odds that a round will tell the intended story. If its too low then the story never fully develops and its not interesting. If its too high then the player characters lose their sense of autonomy and therefore their desire to play. The car has to be reliable enough to make the journey, but not so fast as to ignore the scenery.

Devs tune the car in different ways to get achieve this balance. But CMs become pretty complex and things react to changes in sometimes unpredictable ways. I think they're facing some late-stage design challenges kinda like Feature Fatigue(not a great descriptor but I can't think of a better one right now).

Xenomorphs are hyperspecialized which as a force makes them brittle. They're also outnumbered initially. So their tuning options are somewhat limited and they generally play off of their survive-ability or the effectiveness of their specialty. Marines are generalists so tuning them is a bit more challenging. You'll mostly see things that decrease their fluidity or give them different utilities to explore.