My app runs on android. It does not work without wifi and not even installed on such systems The app connects to a server over WIFI and has to pass it a unique device identifier. The app may have other installations on the same phone/tablet and whenever any of them connect to server they all should pass the same unique identifier.
Mac address seems to be be the best choice given the pre-requisites of running app.
So the question is can I trust that mac address will always be available when WIFI is available and ON ?