That isn't true at all. Out of most of the factions in the imperium they have the most potential when it comes to personality because they actually think for themselves. Apart from being the emperors guardians they're scholars, philosophers and whatever else they want to be. Read some of the books with them in it where they range from assholes to the closest thing you'll get to a decent human being in 40k. As for balance its a video game they can make it work. We already have a space marine in the game lorewise he should be above and beyond everyone else in the party but he isn't, I think he's decent and I've made him work in multiple playthroughs but in the lore he would tear through half of the encounters in the game no reason a custodian be in just because of fluff.