The predators aren't sapient, unlike the Omnitrix aliens.
The aliens inside the watch (yes, even Wildmutt) are all intelligent in some way. They have their languages and can think while using their instincts, while the Nemetrix aliens are non-sapient, they can't reason or think, they solely rely on instinct and nothing more.
If someone were ever to turn into a Nemetrix alien, the lack of sapience would damage their mind and make them act like wild beasts, and the effect could last on the long term if used for too long.
This is why Ben can't become a Nemetrix alien, he'd lose his mind and would be unable to think properly, he'd just become a mindless wild beast
Well in my Omniverse sequel season, Ben 10 Universe Trip, Ben's new watch, the Infinitrix, would allow him to turn into any type of sapient or not species with no limitations, but have good enough wiring to keep Ben from going all animal nuts.