So I looked into this issue, and it seems that if you have 1) provided a WUnderground API key, and 2) selected a specific PWS station; 3) chosen ETo as the algorithm, then you will get a WUnderground Method Not Supported Error. But if you didn’t select a specific PWS station, instead, you just selected a GPS location, then it will fall back to WeatherKit and calculate ETo using that. This is indicated in the System Diagnostics, where it will say the weather data is powered by Apple.
We will be looking into getting ETo and PWS supported. If I remember correctly, partly the issue in the beginning was solar radiation is not guaranteed by PWS, in fact, I just did a few tests, and it’s true that some PWS stations don’t provide solar radiation data (the value is null). But I agree that for those that do provide this data, it should be possible to use ETo. We will look into this.