When people say "generic", it's a less informed way of saying 'derivative'. Derivative art is something that draws heavily (and exclusively) on an already set style without doing anything to make it genuine / different. Consider Elvis Presley: great performer but he offered NOTHING new to the rock / blues genre in terms of technique, style, or innovation. We can all croon about how dreamy Elvis was, or how beautiful his voice was; but that's really it. Compare him to the Beatles who re-defined a genre.
the GazettE are not Elvis Presley. What I mean by that is it's not like they've stuck to the same formula over and over again. Sure their guitar work can be incredibly dry and typical without the saving grace of Uruha's solos (Drop B, 1st and 2nd Frets. There's your GazettE song. Let's call it 'ANGRY SONG'); but they've at least taken the VK 'scene' and evolved in it with their own sound. 80's pop hooks, pop-punk, metal, and electronica all come together to make the GazettE's 'sound', which I will say is pretty unique vs. Elvis.
the GazettE aren't derivative, they just rub people the wrong way. Personally I'm always shifting between thinking they're cool and thinking they're the lamest thing in the world; but that makes me even more excited for this new album: I want to see which GazettE shows up. The cool one, or the lame one.
^ I have a feeling the relationship between the members eroded with all of this and it just wasn't working anymore. Their release schedule was messed up after amp-reflection and, if I remember correctly, they didn't even play many concerts to promote their major stuff. Yumi probably just wanted a fresh start while carrying the same music style from sfp.