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.)