As others have said, this isn't about motors, it's about the VSD. Most work with a very simple and robust architecture on the power side. They convert the incoming AC to DC and then convert this back to AC at the desired frequency (at around 98% efficiency). The electronics to do this are small, efficient, and the design is quite elegant. The outgoing AC can be more or less true sine wave depending on details and need. There is no reason to not do this, so unless you have a better solution, Just run with this. This design accommodates all manner of control ranging from simple open loop operation to sophisticated PLC control.
Options that don't include DC are CVT variable speed belt drives and variable speed hydraulic drives. Both have their niches, but aren't common, are far less efficient, and have to be located near the end point. VSDs are normally located in an electrical equipment room and the variable frequency is distributed to the plant.
Bottom line is that the performance specifications are what is crucial. Just treat the thing as a black box and don't worry about how it works, just make sure it works for your task. That is plenty challenge enough since there are hundreds of different units with different characteristics.