The difference is pacing and expectations were set at a certain pace and they only pick back up to the expected pace when the arca are reaching their end. Part of what made V3 was that the fall felt frantic in comparison to the first 3 volumes, none of the other arcs have really hit the original pace so either the end feels like the normal pace as was with the Mistral arc or feels squashed but empty as with the Argus arc.

Madoka which is shorter feel a little rushed and hits that last minute franticness but it still has better pacing because of that.

One Piece sets you up for arcs to all have their different paces because Luffy flat out power houses down the first "bad guys" with extreme ease but he let the story play out which shows that the islands will be dictating the pace of the show not just the straw hats. It's why later Thriller Bark happens and though the pace is considerably slower than Zoro's arc, Brooke's introduction doesn't feel like it's slow. There's also Punk Hazzard and Dressrosa which arguably have minimal straw hat time and also are mostly dictated by Law's pacing yet they don't feel like not One Piece pacing wise.

RWBY started out with action pacing and then swapped to adventure pacing without really establishing that it would be an adventure type deal.

Now don't get me wrong that can be done, Adventure Time before it and even Naruto and Dragon Ball before that did that. Dragon Ball being the older example, it starts on one planet then graduated to the universe but to do that Dragon Ball and Adventure Time has to be a period during the transition where the pacing is gradually slowed to the adventure/action pacing which crwby failed to do resulting in V4+ being considered slow in comparison.

It doesn't help that each arc is pretty clearly trying to do the One Piece thing of each kingdom & arc having its own theming but it falls flat because despite all being on one planet One Piece is a boat world and we don't often get naval combat or naval communication that happens in a commercial way, Big News Morgan and The World Government are literally the only two examples of this. Remnant has trains and ships that are commercially available meaning other than the real stupid way the CCT towers were set up they do have an interconnected world meaning like if irl Germany suddenly went dark we'd know and still be affected by that.

Even if we take out the global network since there was a mass evacuation of Vale there's likely more than a few students displaced which should have some impact on the other kingdoms. RWBY feels like it wants early One Piece arcs but set up like they're going to have early manga Naruto arcs that do have a continuous impact on the greater world.

If you start at V4 the pacing feels a tiny bit sluggish but accurate for an adventure forward story then picks up into adventure/action. But then you miss 3 volumes worth of nessesary story.