The title says it all. If plastic or fiberglass is allowed by code, what if any cons would there be?
Same cons as any application of a plastic box .vs. steel - not as sturdy, not as good at containing arcs and preventing a fire. Sturdy may come into play quicker with a 14-50 since the plug/unplug forces are considerable.
Code is a minimum, and barring location in an area with stricter modifications, to USA NEC a listed box of adequate size is allowed, for the most part. Some areas (or occupancies) do not allow plastic, but if plastic is allowed it's not generally limited to a particular size of outlet (but the box has to have suitable entrances for the wire/cable/conduit being used. And it does have to be of adequate size for "box fill" based on number and size of wires, and devices connected to wires. That takes a big box for wires typical of a 50A circuit.)