I have one bad experience with the unhappy ones but that is not my main concern in DIY.
Me and you both know that you don't become a competent installer over night.
And among those who get the experience have a very strong sense of doing it right. By the time an installer reaches his peak in skill it is no longer just a pay check it's almost a calling. He get's to know all the nuances of the duct system when paired up with specific equipment..
I can't say the same for a DIY job no matter how outwardly intelligent.
I am afraid a DIY , no matter who, will only get a job that is "Just good enough" because there is no substitute for experience and love of the hvac business, hence my cornball screen name Hvaclover.