>>1590483The power you put into the pedals can either go into rolling resistance, air resistance, or gravity. So, if you know all three, you have perfect power estimation. Gravitational potential energy is extremely easy to calculate with good accuracy, but air resistance and rolling resistance are harder. By riding up steep climbs, you maximize the power going into overcoming gravity, and minimize the power going into rolling resistance and drag, making the estimate better.
The discussion of speed^2 and speed^3 concerns the degree at which the drag term is proportional to speed. Because the "drag power" = k* speed^3 (for some "k"), any error in picking "k" will cause the "drag power" to be wrong by a large amount, when speed is large. You can compare this to the situation of "rolling resistance power" = c * speed. For example, if speed is 1, then "drag power" = k, and "rolling resistance power" = c. But if speed is 2, then "drag power" = 8*k, but "rolling resistance power" = 2*c only.
>>1590486see pic related
