no used or vintage amp can really be really really reliable. can have great tone but you'll never know what will go wrong over time. everything can be repaired though so there's no major loss.
It's great that there are so many lower priced amps around for players nowadays, but they are lower priced for a reason. A good portion of them are made to be disposable. They are either a pain for a tech to fix, or unfixable. And half the time they'd cost so much to fix that it's not worth paying for it. Just go out and buy a new one when it breaks, which is exactly what the manufacturers plan on you doing.
My Princeton Reverb is from 1968. The only thing I have had to do to it since I have owned it is replace a pot, and buy new tubes. It's been going for for 39 years, and it will likely go for another 39 with repairs here and there along the way. It's built to last.
Things are way more likely to go wrong with that $500-700 new amp you buy for the exact reasons that Wild Bill mentioned. A lot of them are a disaster waiting to happen. You can buy a vintage Traynor, Silverface Fender and a lot of other options for the same or less money and get a quality amp that will be serviceable and last.