Also not a pilot, but a lifelong aviation buff: It's not safe, but it's a lot less dangerous than many other things that can go wrong. A landing with main gear locked and nose gear misbehaving is pretty survivable; a belly landing is dicier, but still well within the "everybody walks away" range of likelihood. And if it really is just a single missing wheel - well, that's why there are two of them on each axle. Again, it's not the day you want to have, but if that's the only thing wrong then chances are good of everyone walking away.

edit: That said, as noted elsewhere in the comments, it's pretty pointless to speculate, especially when a linked article mentions more issues than just a wheel having come off. (Although it's not unreasonable to notice that in particular, seeing as an Air Canada Dash-8 dropped a wheel on takeoff just last month.)

A single engine landing with no nose gear is definitely trickier since you'll lose steering authority as the plane slows down--more danger of bouncing off to the side and flipping over, but overall I'd expect everyone to walk away from this with no more than minor injuries. The minor injuries happening when people have to use the rear escape ramps that are at too steep of an angle because the plane is nose down on the ground and the tail is way high in the air.

Chances are they'll have that plane back up in the air in a few months in regular service unless it was right on the edge of retirement already.

Common risks are Aircraft skidding of the runway, fire in the wheel well.

Mitigating factors: Rear wheel brakes should still work. Reverse thrust can further help slow down the aircraft.

Given the response time of emergency evacuation + fire response teams, likelihood of mass casualties are low.

Need not be a big problem, some planes landed missing a wheel without even noticing.

https://avherald.com/h?article=4c29e455&opt=0

https://avherald.com/h?article=4b92a56d&opt=0

https://avherald.com/h?article=4b4af152&opt=0

https://avherald.com/h?article=4a7d012c&opt=0

According to Air Canada, it was a tire rupture (one of 10) on takeoff and the emergency landing is due to an engine issue. Presumably the tire entered the engine.

"A spokesperson for Air Canada told Sky News: "Air Canada flight AC837, Madrid to Toronto, experienced an engine issue shortly after take-off. A tire also reportedly ruptured on take-off, one of 10 on this model of aircraft."

https://news.sky.com/story/air-canada-flight-returning-to-ma...

Gear up landings aren't that uncommon in smaller planes.

A similar 767 made a gear up landing in 2011 with no injuries. [0]

[0] https://en.wikipedia.org/wiki/LOT_Polish_Airlines_Flight_16

I'm not involved with the airline industry in any way, but I would think that since landing gear is a highly mechanical, moving piece with plenty of failure points, airliners are probably designed with a margin of error to withstand gearless landings without loss of life.

This is absolutely the case. Both in cases where the gear is semi-functional, and when it is not functional at all. For reference, here is what a belly landing looks like on a 767:

https://youtu.be/UC8ySY_GlUk

Not very dangerous, they have procedures for this. Depending what is missing they may do a belly landing or if it’s one of the main gear they might even be ok to do a gear down landing.