1) At the time the ET algorithm was implemented, solar radiation was not a required field by WU. So while some WU stations report solar radiations, a lot didn’t. I am not sure about what’s the current situation, whether it’s required now or not, but if you are able to host your own weather service, then
2) You can modify the weather code to allow using ET with WU. In fact, users who host their own weather service primarily because they want to customize the weather code. So what you were asking regarding the ‘difference’ is mainly that you can modify the code and enable ET with WU, or you can customize it in any other way you want. I am not familiar with Ecowitt. The OpenSprinkler weather code:
https://github.com/OpenSprinkler/OpenSprinkler-Weather
supports several weather providers, it’s relatively easy to add additional providers, as long as you have their APIs.