In 50 years of doing car work, I've done just about everything at least once. Except mud work and paint. Will not do any of that outside of a minimal amount of work on a repair. Windshields and back glass are left to glass guys.

Degree of difficulty depends on how bad the car is and who has worked on it previously. And the availability, or lack of, proper tools and equipment.

We had guys pull Camaros out of fields and drop them off at the shop with snakes and mice crawling out of them, and not much good original metal left. Paid good money for it since they watched TV and "know what it's worth". And they thought they had something that shouldn't take much work, until I started walking around it poking holes in it with a pocket screwdriver.

Know when to delegate. Upholstery, windshields and rear glass, mud work and paint, etc, someone else can do that. Someone who does it for a living and could do a better job than me with less headaches.

The worst...stuff that some one else has butchered up before I get it. We had a '67 Chevy II dropped off once. Running, driving, looked okay from 50' Guy want a mild restomod done. Had been wrecked and had rust repair done multiple times. Had 3 partial or full quarters done on one side and two on the other. None were done well. Not much left once the trash and mud was cut off.

Owners who don't know what they are doing, but think they do. Owners who think they should get $120K worth of work for $20K, tops. Owners who think their car, that's not much more than a brown spot on the floor, can be "patched up" and most of it reused. Owners who want to use China trash off Amazon. Owners who watch TV shows and think it takes a week and a half to build a car.

Guys who say "No one will ever see it".

If you can stay in your shop and do nothing but your own stuff, you're fortunate. If you can just do work for people who know what it takes, wants it done right, won't nickel and dime, and drop money off when you ask for it, you're fortunate. If you have to take in everything that comes along and work for clowns who want first class for crap money, and want you to patch and halfa$$, it sucks.

Buddy of mine has a shop. He's right up front. We do it my way, which is the right way with good parts and materials I spec, we don't cut corners, and we do it for this much money. Not negotiable. He sends plenty of people up the street. He's also booked out a year and a half with people who want to pay him, and wants me to come work for him.