Your insurance company would certainly prefer X and Y rated capacitors but... so long as your house has decent circuit breakers, then metal box around them should be sufficient to not essential allow the failure to catch fire to your carpet or whatever. Decent circuit breakers will pop instead of allowing an electrical fire.
X and Y are just guaranteed to fail in a certain way. I use them, noise specific ones, but Audience is methodology is to cover as wide of a spectrum with the least capacitance. That is not what I do.
The better the capacitors, the better the accuracy, the better the coverage, essentially trying to maximize the theoretical capability of a give capacitance amount. At least I think that is the intent... maybe capacitors are easy to market with big price tags?
