The trouble I think is that modern trained people don't realise the mechanical work that needs doing, and linear recording systems of every kind suffer from this because young people are trained now by slightly older people who also have no history and memory. I used to be rather good with Ferrograph series 7 and Super 7, but I stopped dealing with them then, so the really clever ones passed me by. Equally, the old series 4 onwards machines and early Grundigs/Philips that I used, but never had the covers off I would not be competent with. Look at the pinch roller comment again - The expert flips it and discovers normal easy movement or resistance that should not be there. If you have never known a good one, you cannot speculate. The Ferrographs were not easy to trick and some functions were interlocked so unless you know that the Dolby unit will NOT engage if the speed control is set to 15IPS, it's not your skill at fault, it's memory and experience. Heads again. Some brands had by design flatter faces that could be mistaken for wear, and others have (or should have) perfect curvature with no flat spots. Head gaps again - I've seen people assume a head is worn when it's meant to have a wider gap than the record or replay. There will be people who don't subscribe to the cap replacement plan. Especially those with capacitor testers, which often say 'good' when the expert knows about borrowed time. A modern trained engineer might be really competent with a quad trace storage scope and have the ability to cope with leading edge distortion on digital systems but be totally at sea with servo speed control.
I'm not defending rubbish repairers, but my feeling nowadays is that if you want vintage equipment, then you need to learn these skills for yourself - like the vintage car people do. Audio repair skills can be picked up with effort, and many old books and manuals are still available. Expecting skilled service, even at a distance is going to be impossible soon.