[Matrixbeast]

It comes with the drastic trade off of seeming less like a powerful form, and more like someone trying to one up each others' OCs.
"Black RX has limitless power under the sun!"
"J has unlimited power from the Earth!"
"Yeah, well...Kuuga can just...Blow up the Earth, so he's best!"

I'm a fan of show, don't tell. Kuuga's Ultimate form is the epitome of the exact opposite. We see it in one anticlimactic punch out that he barely scrapes by in.
Meanwhile, we're told that he can just casually destroy the world. Additionally, it's only fight was against a villain who never displayed their power, either, as it was the only time Daguva participated in the plot.

Kuuga's finale hinged not on how strong the characters were, but how strong everyone else said they were.

Ultimate Kuuga's Ultimate Kick can wipe out the world and N-Daguva-Zeba killed off most of the Grongi tribes.
There's absolutely no evidence of either, but as long as someone said it, it has to be true.