I've replaced many chopper transformers over the years - but it is fairly rare, and it's only certain specific models where they do fail. It's
VERY, VERY rare to get a 'one off' failure - if you come across a transformer failure in a particular unit, it's most likely you're going to get lot's of those units in with the same fault.
Faults I've seen include O/C windings, shorted turns, and even shorts from primary to secondary
From what I can remember we used to stock 3 or 4 different chopper transformers?, so that was the sort of number of common model failures we saw here in the UK.