What you are prepared to do, when it is for oneself, is very different to what one should attempt as an employed expert. I do all the plumbing, joinery, building, decorating work, plus anything else which needed doing, here - but I would never do it any of it for anyone else, claiming to be a professional. As said, horses, for courses, exactly as the OP has found.
If my home needed ductwork, for a cooker hood, yes I would do it, but if someone came to me and asked me to do the same job in their home, I would suggest they go find an appropriate tradesman.