Being repeatedly hit in the head at full force is not good. Most sparring is done at considerably less than full strength for that very reason. I think MMA fighters take less of a beating to the head than boxers because the fight is more varied, and seemingly paradoxically, since the gloves are smaller, if a solid punch gets through, you're more likely to get knocked out. Boxers, with the bigger gloves, take more a continual beating to the head.
If you go out from a chokehold, it's either because your training partner is an asshole and didn't let go (unlikely in my experience), or you were stubborn and refused to tap (more likely). The seizing is typical when coming back from a blood choke - not good, of course, but it's what happens.
Going out in a chokehold isn't really that bad. I definitely agree that you refusing to tap happens a lot more than anything else. Note that all BJJ chokes are blood chokes, not air chokes, so they work by cutting off blood to your brain. Since this only happens for about 15 seconds or so, its pretty safe. Also note that the "seizure" is nerve stimulus response after the brain stops control -- its not a grand mal seizure in the medical sense. For the most part, BJJ is pretty safe. The thing that you have to be most careful about is tapping out when someone has the hold. As someone with an abnormally high pain tolerance, I'm always careful about tapping out a little bit quicker, because if you don't a number of the advanced techniques can quickly cause permanent damage. However, this is one of the things that you are trained in for BJJ rolling, just like Judo teaches break falling.
The striking sports are definitely more of a risk than grappling (also less useful in my opinion). Boxing is probably the most likely to cause long term trauma like hematomas.
I remember hearing that the widespread use of boxing gloves actually increased injuries and fatalities as you could punch somebody in the head without having to worry about breaking bones in your hands.
If you go out from a chokehold, it's either because your training partner is an asshole and didn't let go (unlikely in my experience), or you were stubborn and refused to tap (more likely). The seizing is typical when coming back from a blood choke - not good, of course, but it's what happens.