The mutations in the group that I currently GM BC for have turned out quite… interestingly
Our slaaneshi "I win all social challenges except charming a noble woman" renegade got the Illusion of Normality mutation as his first one. Its a good one but also a bad one, since all those fancy Gifts of Slaanesh he will get later wont really give him any good bonuses since he will not look like a avatar of Slaanesh, just like, well, a normal dude. Next he got a Gift of Slaanesh daemon weapon for being present while his fellow apostate failed a ritual to summon a daemonette. Those rolls kinda worked out great, since now we at least know where the daemonette went to.
Our tzeentchian apostate got his first mutation from that first failed ritual, which just so happened to be Featureless Face. That made his social interactions, and well, being the "face" of the group to a whole new light and world of hardship. His second mutation came from a (this time) successful ritualistic sacrifice of the Arch-Bishop of the hive planet they currently resided in to summon a blue horror to his bidding. All went well until he rolled perils from the ritual psychic phenomena roll. Like that wasn't bad enough, he rolled Grand Possession from the perils table. Then he burned infamy to "survive" the possession and I ruled that caused the ritual to summon a blue horror AND a Bloodthirster instead of being possessed by a Lord of Change
Needless to say, the next few hours/days went past in a mixed flurry of "by the gods, we need to get out of this noble hive spire to the lower levels", "did you know we are being followed by a greater daemon of Khorne that is very pissed off at you and is literally destroying everything in its way?" and "it seems that you just opened a warp rift that keeps on pouring daemons to the city". In the end, we all found it very fitting that when he rolled his failure based mutation that the apostate got the Pseudo-daemonhood mutation. So now he resembles a horror-human hybrid that has a featureless face and is unable to communicate to the masses about the glory of the Dark Gods. Luckily he is quite close to getting Mark of Tzeentch and some telepathy powers so he can start communicating properly once again
Also, our Psyker also got the Featureless Face mutation from taking part in that first failed ritual(which also gave the apostate that very same mutation).
Indeed, the Dark Gods(and the dice) are fickle.