I think one can differentiate between balanced and non balanced poly phase sets.
Starting with three wires and any 'non-linear' phase difference between the AC on the conductors, one can use transformers to derive a complete poly phase set. Eg. starting with two l-n 'phases' of a 3 phase wye system, one can derive the third 'phase'. Or starting with true 2 phase balanced power, one can derive true 3 phase power using a Scott T transformer configuration.
But if you start with an unbalanced system, and derive a balanced system, and then place a balanced load on that derived system, then you will see unbalanced loading on the original system.
Consider the '2 legs of a wye being used to derive all 3 legs' and then place a 100A resistance load on the 3 leg system. If I did my math right, you get 173A supplying the two original legs, with a leading power factor on one leg, lagging on the other.
You can similarly differentiate other polyphase but not complete sets.
Jon