- It requires a high volume of rare earth magnets to get good performance. They should be comparing against similar motors with rare earth magnets and integrated drives, which will tend to be very high efficiency and power-dense machines, rather than low cost induction motors.
- Copper losses will be high because you typically can't get as good of a fill factor with PCB traces as with wound copper, so the phase resistance will be high, which impacts the efficiency
- PCBs are also not especially cheap at the large sizes, layer counts, and specialty copper weights demanded. At scale it may be better to just automate a high performance epoxied winding.
- The axial flux design scales somewhat awkwardly. At the inner radius (near the axis of rotation), the surface speed of the rotor is quite low, as is the developed torque from magnetic pressure, so most of the power is generated by the material near the outer radius. At the outer radius, the magnetic flux return path through the rotor backiron is quite long, and the magnetic pole faces are very broad, requiring the backiron to be quite thick. The centrifugal forces also get large there. The inner radius wants to be a high-speed, low-pole-count motor while the outer radius wants to be a low-speed, high-pole-count motor.
- Despite the claims about easy cooling, it doesn't seem nearly as easy to cool the stator as with a conventional motor that has the windings coupled to the outside stator where heat can be directly coupled to the environment.
- The front and back rotor sections have a very strong static magnetic attraction to each other (they want to clamp down on the PCB), which requires a fairly robust structure to resist.