my 5 cents on whitelisted roles
preds: a nuisance at best, incredibly fucking annoying at worst. preds don't really contribute to anything except for their personal enjoyment. I really doubt that preds can ever be made more fun to anyone who's not a pred.
synths: literally assistants but kitted in the strongest armor known to man. synths who realize that they are, in fact, an assistant, will attempt to compensate for this miserable existence by hitting up the frontlines with a fire axe. some also like to build cades and be a bullet sponge for marines who are actually trying to shootother marinesxenos. I personally hate combat synths with a burning passion and legitimately wish that combat synthing was a permanent whitelist removal and a >1 week ban. nothing in this game pisses me off more than seeing synths on the frontline. nothing.
CO: honestly just a fancy XO with slightly more responsibilities. the whitelist just makes the character variety smaller, though it's kinda cool how certain COs get their own rep. there's not much room for making COs unique and distinct from XOs without effectively punishing marines for not having a CO. it's kind of whatever to me, i can live with or without COs.
my 5 cents on whitelists in general
as a dev: I personally have very little interest in developing whitelisted content. I don't want to develop content that's not available for everyone to play with.
as a player: I think whitelists gate content behind writing stupid short stories seemingly just to pad the app, while offloading actually role relevant questions to random people replying to the app.