I have a dual sided Shimano R8000 4iiii power meter. I rode 55 minutes on CompuTrainer and the average watts the trainer calculated and what 4iiii power meter calculated were within 1 watt of each other. It doesn’t indicate that either one is accurate. However, it is good that I will have consistency indoors and outdoors. And for my own personal use, I am electing to believe that both are very accurate.
The left/right balance between CompuTrainer and 4iiii are very similar. On the CompuTrainer even if I pedal with one leg, I still get a reading that power is 35% on the side that has no leg pedaling. The 4iiii, notices pretty quickly that ALL the power is coming from one side.