I stuck with the Reboot era all throughout. I agree that the issues after LSH 100 declined, but I still enjoyed them because those Legionnaires were MY Legion. I grew up with them through elementary and high school; so even if they didn't really do anything, I loved reading about them anyway.
Not so with the Threeboot. I lost interest a little bit after Supergirl left. Usually, I love reading about young heroes finding their niche and becoming effective (that's one of the things I loved about the first 2 years of the Reboot Legion). Yet, it was a mix of these Legionnaires being so gosh darned ARROGANT (I'm looking at you, Lightning Lad and Ultra Boy); having saved the world many times before (like what they pulled off against Lemnos and the Dominators); and somehow STILL not being able to pull together as a team.
Conflict within a team is good for stories; you just know not everyone in a team that big is going to get along. But the Legion is all about teamwork DESPITE those little quarrels. The Threeboot felt like an unending squabble between huge, young and immature egos.