VFD’s are very cost effective for converting ex industrial equipment that already has a 3ph motor. In some cases taking only minutes to get a machine running of single phase. In the home shop I can’t imagine that there is a stand out advantage to one type or the other. The biggest disadvantage to treadmill motors is that they often don’t conform to industry standard frames or lack mounting feet. This is nothing that can’t be addressed by a home shop machinist. Also there are as many 3ph motors in the waste stream as there are DC treadmill motors. Used motors can be surprisingly cheap. Now in an industrial setting I’d prefer 3ph motors every time simply due to the reduced maintenance. In a home shop you would likely not wear out the brushes and if you did you would effect a repair before the commutator is chewed up.