ATS Recorder supports all basic widgets from the modeler and some custom widgets. We try to look at the most downloaded widgets from the app store and support these with ATS. More and more widgets are being supported, this is an ongoing process.
This doesn't mean that other custom widgets don't work in ATS, most of them will work fine when you use the ATS Recorder or ATS helper, but in some cases you might have to create your own custom action.
ATS is not limited to Mendix apps, but has been designed for Mendix. ATS supports widgets provided with Mendix out-of-the-box. If you would like to use it for different platforms, you need to build your own way of identifying page elements on such platforms. This requires custom work.