Q: Does chain stretch affect the power measurement?
A: No. The system calculations are such that chain stretch does not affect the power readings. Even if this wasn't the case, chain stretch is so small that it would be insignificant.

Q: Does dirt or oil on the chain matter?
A: The weight of the chain is used in power calculations. But the chain would have to be coated in thick mud before this is of significance - normal chain oiling and a little grime won't matter.

Q: Does riding over bumpy roads, or chain slap, affect power measurement?
A: No. Chain slap only happens when there's no tension on the chain, which occurs when coasting (0 power). Pedaling over a bumpy road does not cause errors in power measurement, as bumps don't affect the tension of the chain.