ET data is not yet provided from WU, but I did get confirmation from their developers that it is something on their priority list for 2017. I am not sure if they are going to be simply calculating it on their end, or receiving it from (for example) our Davis Vantage console, but they definitely said they are adding it.
Regarding the WU API, I have not really looked at it to see what data it provides. But our WU station does have soil moisture and temperature, as well as leaf wetness data there. So I would hope that their API is able to provide that data to us. It is extremely valuable!
I will check on it and see what we can get … Possibly the API does not return any data if it does not exist as a measurement for that area?