How often does that happen on an XS? I have holed a lot of RD350 pistons, and a few other 2 stroke pistons, but never on 4 stroke engines.
Holing a piston is not common as far as I am aware, but it can happen if you get the timing and mixture wrong.
Burning a valve can also occur if the valve clearances are set too small. The rule I was taught as a kid was: a noisy valve [within reasonable limits] is a happy valve.
Pete