Nice addition, and good catch on the redundancy of the RMSE in this case! 😄

Two small remarks:

• For this type of statistic I think "count" is the name often used, for example the statistic argument of this SciPy function: https://docs.scipy.org/doc/scipy/reference/generated/scipy.stats.binned_statistic.html, or the name of this NumPy function: https://numpy.org/doc/2.1/reference/generated/numpy.count_nonzero.html. We could use this here too?
• It might be useful to some users to know the total point count (NaNs included)? If we report a "total count" and "valid count", then users can derive the percent of valid points themselves by dividing the two.

Last thought: I didn't see any function to calculate the valid points in the changes, maybe it's not there yet! In that case I would recommend np.count_nonzero applied to np.isfinite, instead of ~np.isnan, as the latter considers only NaNs but not +/- infinity that are often unusable in stats (and I've had some misadventures with those being propagated in raster data before!).