[Pazuzu]

I've gotten a few older mechanical watches running again, mostly by brute force and general mechanical know how. I do NOT go as far as removing bridges, etc.
Lots of "broken" watches from the 70s and 80s are simply stuck, or have a broken stem or something, and LOTS of them can be rebuilt with new parts. I got my MILs old 80s Tag aquaracer watch going again, I could have completely rebuilt it with new parts (movement, face, hands, stem, crown, etc) but with just new hands (the lume had disintegrated), a new stem and some deep cleaning in an out, and it works perfectly and looks 95% brand new.

There's a app called "Watch Accuracy Meter", it uses your phone's microphone to listen to the watch beats and can tell you how far off it is, it's at least surprisingly precise if not accurate.

Also, watches can get magnetized and run like crap, don't use your normal precision screwdrivers and such, they can magnetize the movement. I use a gause meter app to tell how magnetized a movement is.

Watches that run FAST are getting clogged with gunk, the movement can't travel smoothly through the full range of motion, so the balance wheel swings only partially, meaning it swings faster, and the watch runs fast.

Watches that run SLOW are magnetized, the coil spring is sticking to itself and not imparting enough momentum, so the balance wheel takes long slow swings and the watch runs slowly.