In general I would say "YEs they are safe to use" because when you plug in a 30 amp RV or two the wires and cords and such are protected by the 30 amp MAIN breaker inside. Oh there is a very very very slight chance of an issue in the cord itself but even a 30 amp cord can haul 100 amps for a short time (way longer than a 50 amp breaker takes to trip) so it's safe
But plugging a 20 amp device into it.. I'd not recommend.
Rather I'd buld a distro box.. Basically a 30 amp short cord. a "Sub" panel breaker box such as in a 30 amp trailer. a 30 amp main and 3 15 amp GFCI breakers and 3 Outlets one on each breaker OR go 20 amp breakers and 15/20 GFCI outlets.