Building Science Geeks love to hate on flex duct in HVAC systems. They take pictures of terrible flex duct installations in crummy attics and link to this TAMU article which says that if you crush it, let it sag between joists, and force 3X the recommended CFM through flex duct, the result is a drop of pressure in the system ( https://oaktrust.library.tamu.edu/bitstream/handle/1969.1/5275/ESL-IC-06-11-133.pdf?sequence=4&isAllowed=y ).
Well, duh! Any product, when used incorrectly, will perform poorly.
I think the take away should be how small of a pressure drop there actually is even when the flex duct is crushed 45% AND sagged over joists for a long period using REASONABLE CFM rates – 1″H20 per 100′ run. That is way better results than I would have predicted.
When installed correctly, flex duct moves air just as efficiently as rigid duct and is much easier and cheaper to install. Flex also has the advantage of being pre-insulated and eliminates joint work except at the terminations, reducing heat gain and leakage.
Yes, flex duct has to be installed correctly, but so does rigid ductwork.
Keep the flex duct liner tight, properly secured, the runs as short as possible, and eliminate as many tight bends as possible and you will have a great performing flex duct system.
In my opinion, the best bang for your HVAC dollar is to have a professionally designed system that is COMMISSIONED after install to ensure that the performance of the as built air conditioning system matches the designed requirements.