We all know weather isn't an exact science. What Et does is basically uses the weather to guess how much moisture in in the ground. That's a guess upon a guess and proven not to be reliable.
Localized Et weather data source isn't free. And even if it is available, chances are the weather stations are miles a way from your home, there is no guarantee that it won't water in the rain.
Another aspect of Et is that it requires lots of data input i.e. soil type, plant type, etc., which most home owners and many contractors don't have answers to. It makes Et hard to use. Those systems that claim easy Et set up isn't true Et, just another guesser.
If you want to know how much moisture in in the ground, put a moisture sensor in the ground. This is the direction we're heading for.