I think I would agree that you can probably have a much worse incident on bike vs a One Wheel. A bike allows you reach effectively unlimited speeds (provided you can find a big enough hill and tail wind), which brings with it uncapped risk.
But I think for most average bike riders, people who just ride a bike to get from A to B, rather than for sport (more speed always means more risk, at certain point I agree a bike becomes inherently more dangerous just due to the higher speeds). The odds of them having a bad incident on a bike is much lower than on a One Wheel. It’s quite hard to have an incident on a bike that results in a guaranteed head injury (please wear a helmet, it only takes a single bad head injury to ruin your life), whereas it’s quite hard to avoid a head injury from a One Wheel incident. Most bike related incidents (where someone isn’t just riding into a stationary object, in which case I don’t think the mode of transport matters much) result in the bikes wheel slipping out from under you, the consequence is moderately slowed, but quite painful fall sideways. The time it takes the bike to fall out from under you gives you a bit of time to prepare for the impact. All this assumes reasonable low speeds (not much faster than a run), due to non-sport based riding.
On a One Wheel or similar, dismounts tend to result in full bodied face plants. If you’re not able to come of the device, and catch yourself by running, then you tend to go face first to the pavement, and there’s nothing slowing you down. No bike frame or similar sliding across the ground and buying you time, just a whole load of empty air between your head and the ground.
