It's probably fine, but you can accidentally short it when moving it around by touching it or resting it up on something conductive, even if you're careful dust might short it as well, but this is much rarer.
As a general rule I would try to avoid it, but would not be my first concern.
In all seriousness, its gonna depend. If it can survive minimal ventilation, throw a shoebox with some holes at it. You probably dont want it getting touched, but so long as its not, you're prolly fine.
I killed 3 Raspberry Pis by putting them onto a metal surface while turned on (first two times I didn’t know what was happening and the third time was accidental). Do not recommend.
Hell I ran the processor (450MHz P3) without a fan, just a passive heatsink.
They still make passive heatsinks that work for modern CPUs, but they're fuckin massive and not incredibly effective compared to just.. putting a quiet fan on it.
Most cases have ridiculous nonsense for cooling though. Laptops usually have considerable thermal engineering, but kinda set the threshold of real hardware requirements.
I was just given an old "gaming rig" someone didn't want that had water cooling, 9 fans, a cobweb of LED wiring, and the most obese ABS panels attached to the sheet metal case.... The thing works fine even when overclocked with no fans except the one built into the power supply and GPU. Only the one in the GPU cycles on enough to be audible... This guy had a freaking harrier jet taking off in the room beside him for a decade.
If you are statically charged and you touch the bare PCB, you will do damage. Having a case prevents you from touching the PCB directly and a metal grounded case will also allow you to discharge without damage.