Most converters in the past have been static types, designed to operate only one motor. They are not multi-motor converters. As roto-phase is rotating, a phase shift is accomplished through a rotor spinning within an electrical field. The result is a true three phase. Performance substantially exceeds that of all static phase converters - a comparison is almost unrealistic.
Some static types can give a reasonable performance, but even they are limited in comparison.
Roto-phase is a manufactured device that comes closest to a solid three phase line. Comparing with an open delta three phase system the roto-phase is equal and every bit as good. Against other solid three phase systems roto-phase is merely the best substitute. For short seasonal loads or small loads, it is generally impractical. The roto-phase cost is often just a fraction of the cost of extending a three phase line, whether a few hundred feet or many miles.