In the case of pirateweather.net, "Open" is referencing to the fact that the processing and transforms are openly divulged. Read this http://docs.pirateweather.net/en/latest/ it's interesting
I mentioned darksky because you listed it as part of your evaluation and it was not graded good. This means you were not satisfied already with the approach and results in your region at least.
I also care the most about daily and hourly precipitations and how intense they get