I have never started fresh. My backup can literally trace it's roots to my iPhone 3G.
I have the same experience. My backup is very old, iPhone 4, and has been on a 5, 5s, 6, 6s, 7, 8 and now X with all public betas ever available. Never had any major problems that caused reason to start from fresh. I have so many individual settings that I rely on that I wouldn’t wanna go throught writing them all down before installing from fresh.
My iPad config goes all the way back to the first ever iPad. Same experience.
I don’t even remember how old my Mac setup is... many years back. No critical problems ever.
I think there are to many people screaming ”Do a fresh install”. The few problems I have ever had was solved by deleting the app that was trublesome and reinstall it. That has solved 99 per cent of my problems. The other 1 per cent has been solved with upcoming system updates.