I've been an active skateboarder, snowboarder, and surfer for 25 years, and have ridden a One Wheel XR for about three years.
All are different in terms of most of the skills needed to do any of them.
The One Wheel is much more like a snowboarding in soft powder than it is like skateboarding, but even then, it's not quite the same. There's a sort of softish center pivot feeling, where on a snowboard it depends on where your edges are.
Skill wise, you don't need any to make the One Wheel do what's advertised. You do need skill to ride it confidently, but you gain those skills as you use it.
I've had one very bad fall on the One Wheel, and it was bad. It slide out while going fast down a steep slippery/sandy hill, and then nosedived, which sent me cart wheeling, then whipped my heel into the ground, shattering it into 23 pieces. I was in bed for 6 months, and didn't walk without help for 12.
I actually think my other boarding skills made the injury worse. I reacted as if the board was a skateboard, where a sideways slide is something I'm super comfortable with and do on purpose. That reaction was to lean deep, counter to the forces of the slide. The big fat gripping tire of the One Wheel bit in, then acted as a fulcrum with my body as the lever, and I just went sailing.
The advice from the GP to have One Wheel-curious people try a skateboard first is indeed good advice. It won't teach you how to ride a One Wheel, but it will teach you how hard the ground is.
Another good option for new riders is to learn to use a balance board first (like a Vew-Do). At the very least, step on the Onewheel while it’s powered off, slowly raise it to level and hold, then alternate slowly setting down each end and returning to level. If you can do that, you can recover from some situations that would otherwise result in a nosedive.
And another important one: braking. Something that has wheels and can move from inertia will need some mechanism to brake, and to emergency brake, especially on inclined / declined paths.
Here in Barcelona these things are super common. The riders seem to be pretty adept. The problem is more that they don't care about pedestrians, they zoom around while on their phones on the pavement where they're not allowed and zip around pedestrians way too fast.
I don't care if they kill themselves but they're risking other people's safety more.
I know a few people that bought these to commute around town. I always thought they were bat's hit crazy and did try, unsuccessfully, to talk the out of it.
I skateboard when I was younger and generally feel comfortable enough on any board where I can stand sideways. No way in hell I wanted to try these though, the top speed alone masks me way too nervous even with experience skateboarding, surfing, and wake boarding.
I have and ride a onewheel but never skateboarded. It really is a different balance; one-wheeling hasn't helped me a bit for skateboarding skills either.
My take on the safety bit is I would never ride the one-wheel at the top speed the device supports, and would never ride without a helmet or on a road with traffic. It can and will dump you. I've had the battery-out dump (when I knowingly experimented after its low power warnings). Basically if you go faster than you can run out/step and roll out from you're inviting yourself trouble.
You’re telling me people were buying and using these without knowing how to skateboard first?
I thought every single person I saw using these were, at the very least, passable skateboarders …