Honestly there's a simple, if perhaps unfulfilling, answer: being sealed had adverse effects on both enemies and rider powers. It's already kind of a given considering the lack of access to final forms n whatnot.

Roidmudes lost their invulnerability so that any finisher tier attack can destroy or remove their cores, Makamou can now take damage from non pure sound sources, Imagin can no longer freely time travel, Ryuki riders and mirror monsters can't drag things into the mirror world and leave them there to rot anymore, Undead can now be sealed by standard finishers, Worms and Kabuto riders can still hit high speed movement in bursts and without it going into god-dodge, so fast they dissapear territory, etc etc.

It might lessen or do away with series gimmicks, but they're just that, gimmicks, they don't matter in their actual series because the characters in those worlds have the tools to even out the playing field. Sure technically Kabuto and the Worms fight at insane speeds but what you actually get are regular fights with some effects thrown in here and there, and you need something like this to keep things even in a crossover world because you can't rely on writing the plot to suit your needs like it happens with the movies.