The point youre obviously missing is that under normal operation wood is fine BUT if a component fails wood catches fire easily and when that fire has fuel it can burn your house down. A metal frame will not catch fire if a component fails and components by themselves dont have enough material to really burn explosively.
If youre using wood, plastic, cardboard(srs Ive seen a few) or any other easily flammable material to build your frames you are a moron. Its that simple. Electronics fail and when they do you dont want to have them light anything up...
Yup. This. The discussion is "can", not "will", and in the can discussion metal will always be fire safer than wood.
Plastic, though? Depends. I have a 6gpu rig in a 4U rack case, but I also have a 4gpu rig in a milk crate. It's stupid thick, hard plastic. Is it flammable? Technically yes, but literally the whole rig would have to catch fire for it to be a concern (and I keep it on some sheet metal in a concrete basement).
Now the sound dampening box for my L3, however, is built from wood. I do plan to eventually line the interior with sheet metal for this exact reason, but I'm also not shutting it off in the meantime because the risk exists, but is rather low...