I think that a letter of complaint should not be unnecessarily aggressive or insulting: after all, the people reading it are probably not responsible for the failure, but are just employed by the company.
I would recommend to stay away from the agrressive tone in:
Your product is completely worthless!
Your new XYZ isn’t worth the package it came in.
And of the passive aggressive tone in:
XYZ should be called WHY instead, because why would anyone want it?
So the best answer is:
The XYZ broke the first time I used it.
