Bit of fear mongering here.
People have died from single blows but in the ones I have heard of it's when a guy gets punched, falls and his head bounces off say a table or stage or kerb stone etc it's not the punch, it's what your head bounces off on the way down .... otherwise a lot of world star hip hop would be snuff films.
On the other hand, getting blasted in the head over time causes brain trauma. I watch a lot of Joe Rogans podcast and he has doctors and stuff on, they talk about that kind of stuff. (has other people on like he had Gary Johnson pimping his campaign to beat Clinton and Trump. Has fighters on, hunters, comedians etc)
However if it's truly like fight club, it would be bare knuckle, which is so much safer. In boxing and MMA the gloves protect the hands/wrist NOT the opponents head. They allow you to punch so much harder and with whatever technique you like and still be effective.
If you take the gloves off, head shots are dangerous to your hands and wrists 'cos the skull is pretty dense and you have a lot of small bones in your hand/wrist and if you can't punch properly, then you're definitely breaking something. If you look at the wrist part of gloves, it's very thick and rigid to support the wrist from the shock of punching.
You're meant to land a punch with your index and middle finger knuckles, nothing else. So if you can't do that, you could end up shattering the last two knuckles (ring and pinky finger) or your wrist bends wrongly. People often punch with their palm facing them, which means you either punch with your fingers (breaking them and deliver less power) or have very limited range or you slap your opponent and not punch them. (palm faces down when throwing a punch)
So taking all that into consideration, bare knuckle is safer if you know how to do it properly but fight club style stuff is usually under ground fighting, which is as dangerous as all fuck!
It's illegal, so people will take P.E.D's, fixing will happen, doubt there will be any medical team 'cos what trained doc is going to be at an illegal event? Probably run by blood thirsty dumb dumbs.
If you want to fight do it properly. Go to a proper gym and in all likelihood they will get you a safe fight