I use a oval ring on my MTB with a left-only 4iiii PM. 4iiii says that this may result in a 1-4% overestimation of power and I’ve seen similar things reported for other manufacturers.
For me, using a L-only PM is already a pretty big compromise since I am right leg dominant by about 3% on average. In that regard I don’t worry about it since the sources of error are potentially offsetting, but overall I interpret my MTB power data through a more skeptical lens then I do on my road bike due to the possible sources of error.