During a particulalry intense thread on the stupidity of fire breathing on ‘Home of Poi’ someone posted this link, and I thought it may be of interest, especially to Mike Fule who recently delved into the world of full face helmets for unicycling: -
The jist is that helmets with rigid chin bars will save your face, but can transmit the force into a fatal lower skull fracture.
More relevant for the higher speeds of motorcycles than unis I guess.
Wow: die or live with a smashed-in face. Hmmmm, what to do, what to do…
Somehow the idea of using one’s face as a shock absorber to protect one’s brain is not appealing.
maybe not, but I knew someone who lost his life in a motorcycle crash because he was wearing a full face helmet
All safety issues involve an element of balancing conflicting objectives. For example, the many benefits of wearing a seatbelt in a car must be weighed against the risks of bruised or cracked ribs, exacerbation of whiplash injuries to the neck, and even the (remote) risk of being trapped in the vehicle.
Similarly, the extra weight of a helmet can exacerbate neck injuries in an accident, or can trasmit the force to different areas. Wristguards can increase the risk of shoulder injuries… and so on.
In sports, one of the compromises is between enjoyment and safety. An element of controlled risk is part of the fun for rock climbers, divers, motorsports enthusiasts and even extreme unicyclists.
I suffered a very unpleasant MUni fall without a full face helmet. I thought I’d broken my chin, but I hadn’t. I did break a tooth in half, and I needed 7 stitches to my chin. I believe that a full face helmet would have prevented both components of that particular injury, without causing a different injury instead. I could be wrong - I’ll never know - but I was there and I know what I think.
My instinct and experience tell me that most unicycle accidents will be at speeds of less than about 15 mph (25 kph) and most injuries will be chips or cracks to bones, and cuts or tears to the skin.
On this basis, I think a helmet is a good investment, and I personally choose to wear a full face helmet for MUni, Cokering or road riding, and a normal helmet for less extreme stuff.
It’s up to others to make their own decisions.
I would say, in general terms, that it is dangerous to look for evidence or reports which support your own preferences or preconceptions. We’d all prefer to ride with the wind in our hair (while stocks last) so we might be tempted to give too much weight to reports which show how dangerous helmets are. We mostly prefer not to look like Imperial Stormtroopers, so we might be tempted to give too much credence to reports which say that full face helmets are “dangerous”.
Unicycling is not motorcycling. However, if the comparison must be made, note that all GP and WSB riders wear full face helmets, many fall off, and few die. (Too many, but relatively few.)
Aren’t you afraid that the wind will blow off some of your hair? That’s the REAL reason I wear one! 
I’m very sorry about your friend or acquaintance. I have a close family member who died in a car accident, even though he was wearing a seatbelt. Apples and oranges, but just to let you know I’ve been there.
Was this a situation where the rider would have survived with no helmet, or with a 3/4 or other helmet type? Most people who lose their lives in motorcycle crashes do so because of the crash, though is a small percentage of cases the helmet could do more harm than good. The same can be true with seatbelts, especially in side impacts. But this does not mean don’t wear seatbelts. It does mean don’t get hit from the side if you can help it though.
Same thing is true on a motorcycle. The danger is in the kinetic energies involved, and the fact that the rider is relatively unprotected. The best protected part of nearly all motorcycle riders is their head.
As for unicycles, I haven’t seen much use of full face helmets, though their use will surely increase as more riders get into more dangerous terrain. We unicyclists are perhaps more likely to fall on our chins and lower face areas than a bicyclist, who is more likely to come over the handlebars onto the top of their head or similar.
he would most likely have survived with an open-face helmet on, but full face helmets have also saved many people as well when wearing an open faced helmet would have seen them dead.
Some of the things we do are dangerous, and there is no single way of removing all that danger- you have to look at all the risks first, look at ways of reducing them and then see if you still want to do it.