I had one in their teens - I was expecting a drama after peoples advice, it was actually a walk in the park, no problem at all, of course that's only one horse but still it would not put me off if I knew the history.
Haven't done one that old, although have broken ridden oldies to drive.
I will put in a word for the poor sods who are written off after the first attempt goes wrong. The most honest, quietest and reliable pony I have was taken on as a 6yo, described as 'a kicker' and 'unbreakable'. Apparently there had been several attempts, including by a so-called professional.
Yes, some horses are lunatics, and others can be made into lunatics by poor breaking, but many will come around with gentle, confident handling and once they trust you they're a different horse. This goes double for a Welsh.