you shouldn't be able to join a faction because you belong to the faction of free and independent mercenaries, but you definitely should be able to gain one faction's trust by helping them a lot and getting on other factions' bad sides by slaughtering their men. If they had invested in proper Witcher style dialogue based quests, this would be doable. Or, if you manage to complete an objective without being detected, you should be rewarded for it by your fixer with better jobs, whereas if you get detected, the gang starts hunting for you or something. And if you have higher street cred, they wouldn't dare go after you...and so on. Lots of wasted potential.