Here's the thing, and it's both a pro and a con for McGregor... he's used to fighting MMA (duh), where you have to be aware of so much more than just punches - kicks, elbows, takedowns etc... So, his opponents also had to concentrate on that when fighting him. Mayweather doesn't. All he has to watch out for is the fists, and he has a career's worth of experience doing it. The fact that McGregor has outclassed his MMA opponents is meaningless here.
One could argue that it's also a 'pro' that McGregor can now focus solely on defending punches, but at what point does instinct kick in, where he just naturally is watching for non-punch attacks. Wonder if Mayweather would feign some MMA like attacks, just to throw McGregor off?
Either way, I'd like to think it's a snowball's chance in hell that McGregor can win, but as Honey Roy Palmer once said "anybody can get lucky".