The pressure drop of flex duct vs smooth sheet metal is well documented, not an opinion. Basic fluid dynamics. The rest is those regional differences. You should have seen the look of disgust on my HVAC guy's face when he said "those guys would have used all flex duct!"

Of course I am making the same face when your HVAC guy said "nobody can make a duct to get from the closet to the attic". Ummm, that's the job, sonny. You bang the tin until it takes the shape you want... If you couldn't do that around here, you couldn't be in HVAC installation.
But if nobody does that in your market, then that is the answer you would get. I know my guy well as I work with him and I know the dumbfounded look he would give me if I asked "if he could do it." It would probably be accompanied by the phase "does a bear **** in the woods?" or similar. "Well duh."