I did roofing for awhile, and while I personally enjoyed the work, it wasn't my calling.
It did give me a real intolerance for poor (roofing) workmanship. Water flows downhill, except when the wind blows or it freezes. So you need to plan for that. For some roofs, you are screwed before you begin because the architect made an impractical call about the slope of some valley that you can't correct, so you need to put a seamless roof under that whole area before you put on the visible roof. Not cheap, and nobody knows unless you tell them.
As
@EddieWalker and
@Fuddyduddy1952 mentioned, there is something about roofing businesses that seem to attract fly-by-night operators.
There is something about best practices for most tile and cedar shingle shake roofs that got lost between 1850 and 1950. I know that the way most get done around here are a recipe for early failure.
All the best,
Peter