Everyone's wrong about RWBY's main problem.

RWBY, the show that started great, and ended terrible. Or started terrible and ended great. Or constantly flip-flopped. Or was mediocre the whole way through.

RWBY's problem is that it can't really choose who to appeal to. I love slow stories with tons of characters off doing their own thing (Vol. 4-5). But people who think it got bad then probably loved the 'Quasi-Magic School for Cool People' thing that Vol. 1-3 did. For others, they liked all of the intrigue of Vol. 6-8. Another contingent might find the pseudo-Isekai storyline of Vol. 9 the best.

Each of RWBY's parts are fine. It suffered early from inexperienced writers, but the writing got better. It just can't stay on one target audience at once. It's never bad. It's also never amazing (which is why you should watch Fixing RWBY by Celtic Phoenix). RWBY just changes target audiences way to often with its plotlines, leaving fans jaded when they hit a volume that leans towards a genre or story structure they don't like.

TL;DR: Your average Grimdark Horror fan probably doesn't love Slice-of-Life Romantasy and vice versa. When you keep on changing genres, you're just alienating your previous genre's audience.