I have one of these (Dave Smith Instruments Poly Evolver) and it has about 70-80 encoders. After a while (even with limited use) they start going bad and start getting really erratic. I can treat them with DeoxIT and that does work for a while, but in the end they get erratic again.
A few hundred of those synthesizers (and the related Prophet '08) were made and they all pretty much ended up with malfunctioning encoders. It probably cost the (small) producer quite a bit of money to placate furious customers (and some image damage to boot). They've switched to pots a while ago, even though encoders (arguably) make more sense for this type of application.
Ofcourse issues like these are in no way limited to one producer of goods. My Waldorf Blofeld synthesizer has 7 encoders and they have problems, my (rather expensive) coffee maker has one encoder and it has issues, etc.