The way that a VFD draws power is different from a motor, and can pose more of an issue with an RPC. A motor draws sine wave current, even though there will be a phase displacement(power factor). The VFD generally draws current as shorter, higher current pulses, not sine wave current, although 3 phase input reduces that somewhat*.
When you run the equipment at near full load, the pulses may be equivalent to a 10 or 15HP unit, even though the average current is correct for the 5HP. So the VFD may act to "pull down" the voltage of the generated leg more than a motor alone would. Even if the VFD does not fault, that may put more of a strain on it.
* The reason for this is that the input voltage must exceed the voltage on the filter capacitors before there can be any current flow through the rectifiers. So current tends to flow in pulses which occur at the peak of the input voltage waveform. Since the current flows for less time, there must be a higher current for the time that it does flow in order to equal out to the right rms current.
