I have a similar ac drive for my 3 phase Delta jointer and its great. One other benefit is that if you install a twist lock plug somewhere between the machine and the ac drive, you can start looking at other 3-phase machines and they are generally MUCH less expensive than single phase.

There are 3 pumps each with a capacity of 1000 (arbitrary units) and one of these had an ac drive. If the required flow is 2500, 2 pumps will produce 2000 and the ac drive pump will produce 500. If the required flow is just 1300, 1 pump will produce 1000 and the ac drive pump will produce 300. If only 700 is required, the ac drive pump will produce the 700. The scheme would therefore save energy.

The downside my supplier said is that it's not reckoned to be a good idea to run a cheap eastern or old motor with one, presumably because of the insulation and other extra stresses it can result in if run over or under the stock RPM.

It also happens that for the majority of single phase to three phase vfd drives that don't step up the voltage (i for example am getting 3 phase power at our single phase 220V – instead of the usual 380V) you need a dual voltage motor connected in delta. This isn't an issue for most modern motors, but older and cheaper motors may not have this facility.