They're well worth the almost nothing you pay. (At least 80% of time when they're not straight-to-garbage junk.)
The old thing is missing manuals.
Permanent risk are damaged discs/cartridges (depending where you buy).
Modern problem are protections that may keep the game from installing or playing (which may be technically illegal from them, unless they've been able to pull off the "buying this disc you're not actually buying a game, you're contracting into a licence to use it").