I don't believe in this.Many martial artists critize Krav maga and other war related arts as being innefective.I belive an elite grappler can beat anyone.
Marines/Specials Ops are trained to kill and survive. Fighters are only trained to fight. You must remember this. There are certain behaviors that military guys learn that are highly unlikely for "fighters" to learn.
I'm not saying an elite MMA fighter would be just one-shotted or put into a hold that puts him down in a few seconds ( well if he wasn't blindsided of course ).
But in a 1 vs 1 fight, I'd bet my money on elite trained Marine rather than elite trained martial artists, and to add to it there are even Marines/Special Ops who take MMA classes on top of their military training so the blend of it is even more deadly.
This is my personal opinion. Although it's not like there won't be a case where MMA fighters would put down the well-trained military. I'm just saying that if I had to trust someone to help me I'd go for the Military one.
tl;dr I should've not say they would brutally beat MMA fighter, just reduce it to beat.