Synchronous and asynchronous brushless can both be AC and DC motors. Actually both are 3 phase controled, the difference is only within the rotor. Th e one is a magnet and the other has shortcut windings.

The 1st brushless motors were sensorless which means the stator coils will act as sensors to sense the rotor position. At low rpm there is not enough conduction voltage produced to act as a sensor signal and that is why cogging starts. People did not want this and so later the sensored systems were introduced where HALL sensors pick up the rotor position.

I always said the sensored motor is the worst for stockracing. tolerances from factory as also "made" by the users on the sensors can make a wide range on performances on the motor production. Now these days sensorless controlers are pretty good in low rpm but no one will use them. Sensorless motors are much easier to make and to seal with an equal performance because there is no need to open them and there is no outlining of sensors.

The idea of an asynchronous motor I like. If more toque is needed it will run a lower output RPM than the RPM is given by the controler so no matter the FDR, the overal performance will probably more equalized + no sensors are needed and a full control from 0 RPM because the ESC has only to produce a rotating field w/o sensing the rotor position. But that will give a more demand on low air resistance and drive train resistance compared with normal brushless systems.

I always had the thought to use a sensorless system in low RPM range as a asynchronous system. In that way the startup is full controled w/o cogging and at a certain RPM then switching over to sensing rotor position. To be sure the rotor stays synchronized with the rotating field a soft start could be programmed so the rotor will follow the rotating field.