My dad was a qualified mechanic in his earlier days, so my brother and I grew up with our dad always tinkering on cars, which rubbed off on us. He has a MkII Escort RS2000 and I have my GTA. We both do all the mechanical work on our cars as much as possible. It's all only nuts and bolts!
The only limiting factor is having the tools and facilities for certain jobs.
The only jobs I've had done for me are a clutch change (engine out job I don't have the space to attempt) and cylinder heads reconditioned when the head gaskets went - although with hindsight I probably could have done that better myself, It came back with the timing chain out by one tooth!
And I don't bother on modern cars with anything to do with electronic gizmos. But then garages don't either. They just plug a new gizmo in and charge you a fortune!