In real (arranged) fights, as mentioned above, either the nails are trimmed or the fighters try as best they can to avoid scratches. Having real scratches on your arm or face isn't easy to just explain away to family, friends, or co-workers. Like you can't walk into the office with a black eye.
In fantasy all is good but as far as private arranged fights ( in my experience) nails are avoided.
In spontaneous fights, everything goes.