It’s all about how they adapt their game to fully encompass all aspects of fighting.
Obviously they would do good on striking exchanges, but I have trained with regional champs who have pieced me up on the feet sparring but when it game to grappling they become a fish out of water.
There are also little details about changing a pure kickboxing game over to MMA that really matter.
Such as the glove size. It’s much easier to block strikes with big gloves and that can be a physical tendency for a lot of kickboxers/boxers to compensate that few inches of space with their glove. Rather with 4oz gloves, it is much harder to shell up and leaves a lot more openings.
An example would be to watch Gokhan Saki UFC fights, he doesn’t cover up properly to accommodate for the glove size.
But With that said, I’ve seen plenty of guys adapt high level Muay Thai to MMA. And seeing a top class stadium fighter do well would be plausible as long as they learn how to encompass grappling and tweak their style for MMA.