There are other components that fail, but most of the time is a dead chip or a few of them, if you think that is a small problem which can be easily and cheaply fixed then you need to do more research, the average fix for a chip would be in the $100 range including labor, of course it depends on where you live, but if you are going to buy a used gear for $400 today, chances are by the time it needs fixing you can buy it for $200, fixing a single chip for $100 or even $50 does not make any sense, so more often than not and especially with the old gears, once a single chip dies, the whole board is rendered dead as far as logic is concerned.

Another common issue would be the PSU, many gears are known to have fragile PSU, and the problem is, most gears today don't use a universal 12v PSU which you can buy of anywhere, they rather use their own designed built-in PSU which in most cases are hard to find, many people have perfectly working miners that need PSU and can't find them.