older encoders safe to use?

My first look at a BEI encoder circa 1997 surprised me with the amount of circuitry inside it -
quite unlike the US Digital ones that I had been looking at up to this point, which contain
little more than the disc and the led/phototransistor module. This makes me wonder what
the expected service life for used industrial encoders might be, and if they can be relied
upon. It seems likely that many of the rotary encoders on ebay are around that vintage, or
quite possibly older.

Encoder failure could create quite a bit of havoc, no? Can anyone comment on their
experience with older used encoders - good or bad?

TIA

Steve Clay-Young