PVCx is a natural fire retardant, metal can cause arcing and shorts To be honest I don't think it matters about the rack material, inspecting every piece of heating every wire and connection every few months is likely a great way to prevent problems. Yes it is a ton of work but...

Fail safe thermostats. My belief is that any and every thermostat can and will fail eventually. A fail safe protects the animals in the event of a failure and protects against over heating of the flexwatt and enclosures as well. I don't believe it is something that should be over looked. that is the best use of a hydrofarm to my mind it is almost made for it.