The downside is that it is hard for them to get assigned good quests that wouldn’t go to the Legions, they also can't complain if their talented personnel get scouted.
The plus side of Legions is that it's easier to get good quests and being able to maintain a standard level of competence by way of enrollment tests. Also, they can establish terms for leaving the Legion and regulations, inflicting penalties if those are violated. If the violation is intolerable, the violator can at worst be banished from the Adventurers' Guild.