In the tests at the university the factory variable speed units (GE/Regal-Beloit) do work but there cost benefit ratio including replacement at a given point and time does not balance out in the customer favor. Parts warranty or not since labor has to be calculated into the mix. Almost all manufactures give a 5 or 10 year warranty so that is not a problem even with the lesser priced equipment. Service is also a problem since most service trucks are not equipped with the proper tools/spare motors and electronics or manuals to properly test out the entire circuit on the variable speed DC drives.
Variable speed drives of this type also have a difficult time if asked to work out of range as may happen on a replacement system with restricted or extended duct work. They bounce up speed and down trying to find a comfortable speed during this time of additional stress. I am not going to write a book but I have had the opportunity (as a gift from GE) to equip my entire home system (all 3 air handlers) with there variable speed products for free as a demo. I passed on that offer and still stay with my Fan Handler/A-1 style units with Baldor motors.