VFD and IMO are not going to be competitors in the same market. The benefits of VFD are various depending on the intended use. Bigger the motor (from kW to MW), bigger the benefit. They can act as soft starters therefore reducing the stress caused to the motor during the startup. With the VFD you do not go above nominal current (if you do not want).
VFD for fan and pump applications will considerably increase efficiency of the system as reducing 10% the top speed, the power absorbed by the grid (power is in this application proportional to speed^3) is 27%. As process normally do not need normally 100% speed, you end up saving big money every year.
VFD efficiency for Voltage Source Inverters is higher than 0,98 (without an input transformer); if you add a transformer you need to consider its efficiency;
Multilevel VFDs do have near sinusoidal output (no need for filters towards the motor); in this case are used to retrofit old motors.
The same VFDs are often Diode bridge front end and do not normally go below 0.95 power factor (considering an Xcc of input transformer equal to 8%).
In the end VFD and IMO's are both good in their respective application.