I agree with Tepples. Expanding on it a bit more, as well as presenting my own opinion:
The drives use a very specific kind of belt, made from a somewhat odd or unique rubber (or rubber-like mix), and are *extremely* sensitive to belt length, belt tension, any cruft/junk on the belt motor (this can happen if/when replacing an old belt), etc. -- all which affects overall timing (and you already know what happens if you get this wrong). Replacing belts in the FDS, as I recall, was a common thing. So a lot of FDS drives start acting wonky because of the belt. Can we agree on that?
What affects the belt tension, length, etc. over time? Lots of things -- one of which is heat. I speak from experience on this one. The very first FDS I ever had was given to me by a friend living in Japan who found it in a storage closet of a small company he was working for. Once I got it and confirmed it didn't work (always got an error when reading from disks that were definitively good), I opened it up and found the drive belt had literally melted sometime long in the past. It wasn't goop -- it had melted and become rock hard, and was all over the inside of the system. Cleaning off all the residue was difficult, but even after I did so and replaced it with a supposedly authentic belt, it still didn't work. Adjusting some of the timing pots didn't help either (and I didn't want to mess with those too much anyway, it can actually make things worse if you don't know exactly what is wrong with the system. It's better to leave them alone!). I ended up giving said FDS to Matt Conte (of Nofrendo/cajoNES/nes6502/Nosefart fame), saying "it's yours, I hope you can fix it!" Unsure if he ever did. I got a new FDS sometime later -- same disks, works fine.
Another thing that certainly would affect the belt would be humidity and environmental changes (consider moving from, say, Okinawa (warm part of Japan's southwestern islands) to Hokkaido (cold part of Japan)). You've also got "general environment", like where people placed the FDS in their homes during use, if they left the system on for long periods of time (internal heat), etc.. Japanese homes are not known for being spacious, things tend to be optimally positioned/placed to fit perfectly in small nooks and crannies.
This might help explain why Disk Writer kiosks seem to be of "better quality". Were the drive drives in said kiosk *actually better quality*, or is it simply that the kiosks resided in department stores which were air conditioned (proper humidity/temperature balance), thus the innards lasted longer? Maybe they WERE better quality -- the only way to know for sure would be to have an actual hardware engineer do a teardown of both a Disk Writer kiosk as well as an FDS and provide an analysis.
Anything else is purely speculative/anecdotal (including my own above theory). The closest I've seen to such
is this article from Chris Covell, which has a photo of the innards, but the disk drive appears to be enclosed in a metal box.
My $0.02, likely not worth much.