Western Boxing is the absolute worst because of the fight length. Bar-none. And I say Western Boxing because Muay Thai and Lethwei are short, comparatively speaking.
It's actually kind of stupid. The head trauma a professional boxer takes in a single 12 round fight is staggering. And for what reason? To go to a 12 round decision even though EVERYONE knows who's going to win by the end of round 5. Yes, there are exceptions, but they're actually quite rare.
That's not even factoring in the damage these guys take in sparring, which is what fucked Mickey Rourke up so badly.
I remember when the Conor/Floyd circus was in full swing and an interviewer asked Jim Lampley what he thought Conor's chances were against Floyd:
And I quote:
"Conor McGregor has spent a total of 21 minutes on his feet in his last TEN fights. He's about to go 36 minutes in ONE fight against Floyd Mayweather."
In terms of sheer brain trauma, Western Boxing is unparalleled. The reason we occasionally hear about deaths in the boxing ring, and almost NEVER hear about it in MMA, is because some of these boxers are rolling into round 8 fully concussed, and possibly bleeding in the brain.