IMO they are technical issues. Overeem is a great fighter on the front foot, give him time & space to pick his shots and he'll land vicious strikes with incredible accuracy. But defensively he's just crap, throw more than a couple strikes at him at one time and he starts going to pieces. He'll put the earmuffs on and get backed into the fence where he gets teed off on and finished or run for his life while trying to reset. This happens every time someone an opponent puts some pressure on him, I don't think it's mental, it's bad habits & technical flaws.
I don't follow much outside of the UFC so my list is UFC only.
Max Holloway's results speak for itself. His strike selection, ability to make adjustments on the fly, strike effectively at any range going forward or backward, and use of feints, timing changes, and setups makes him the total package.
Valentina Shevchenko is my other pick. She uses many of the techniques & concepts that are seen only at the elite level in men's MMA and she's doing things which the men haven't figured out yet, like how to counter the push kick to the knee. The parries into counters made famous by Garbrandt? She did it to Holly. Hand traps & guard rips used by Overeem, Saki, and other Dutch kickboxers? She uses those as well. She still hasn't fully adapted her striking from Muay Thai/kickboxing to MMA, but if she does her division is fucked.