No imbalance. For a 220 load, each hot acts as both a neutral AND a hot (the current ‘returns’ to the electrical box in an Alternating sequence). In a 110 load, you need the white to provide a return to the electrical box. I have several circuits wired this way, and it is per-Code.
Kelly – I’ll defer to you on this. Suffice it to say that any 220/240 load has to be supported by the same breaker.
Brian – if you run 12/3, with ground, you have two hots, the neutral and ground; so the two hots support the 220 (the neutral is not used); the 110 is supported by one hot and the neutral..