I don't really agree with this. you think Mayweather had an easy time with McGregor you should see a match where they both grapple.
an mma round is brutal (potentially at least) the first time you get kicked in the leg you gonna wish it had been a punch in the face instead!
I've done both striking and grappling and actually mixing them together, I can tell you without a doubt the more tiring aspect is whatever you are less good at....