Getting hit never bothered me that much. Having your heart come out your mouth while you're trying to calculate your strategy and analyze the strategy of the guys around you while you're going into a hard 90 degree left turn at 27mph with guys leaning on you is pretty d@mn challenging.

Where I will give credit on boxing is taking/giving the punches, while analyzing strategies, and expending a lot of energy is tough also. Same with ultimate fighting. I can only imagine though - I haven't actually done either. The only martial art I've done is fencing (its way more demanding that you think also)