Lio Rush
While the likes of Harper and everyone else on this list have legitimate gripes with WWE, Lio Rush might be part of his own problem. He might think he's getting underpaid, but that's the case of everyone who isn't one of the top 10 stars in the company.
He thought he should have been pushed as the top star of 205 Live and also sees himself as the biggest star in the company. That just isn't true. His confidence is both a blessing and a curse because it got him to this point. But it also has him hitting outside of his weight class.
Rush is supremely talented but is unfortunately vastly undersized in a company that greatly values it. He left Ring of Honor rather quickly, probably due to his perceived value as a top star in the industry. Rush has also asked WWE to release him rather than send him back to NXT. He hasn't been on any WWE programming since before WrestleMania.