Always baffles me this, how people have so many issues with gears.
Setting them up correctly in the first place is pretty straight forwards and after an initial adjustment on new cables there’s very little to do on them.
I think that people overestimate just how fine an adjustment is required to get them shifting sweetly, the same also applies to disc brakes. Line them up correctly and so long as your rotors are striaght you should have very few issues.
Personally I find that with gears, XTR cables in SP41 outters is faultless and cheap enough, sling on a KMC X9 and above chain, get them set up correctly and they’ll be fine for months on end.
Brakes, it’s more than loosen the bolts, pull the lever and re-tighten the bolts. They too require careful alignment and so long as the calipers are in good working order – pistons move in and out tangentially to the caliper body – they’ll be fine.