Are there known bugs or issues with getConnectionInfo().getMacAddress() when it's used as a unique device identifier?
One of the alternatives, the Android ID, seems to be not available (or not really unique) on some tablets. FAIL...
The only problem that I know of is that it is only available if Wi-Fi is enabled.
I think this is a big problem because some smartphone users probably never turn Wi-Fi on. Your app can turn Wi-Fi on itself, but this means your app needs an additional permission "android.permission.CHANGE_WIFI_STATE".
Your general issue - how to uniquely identify the device - is widely discussed, e.g.
Is there a unique Android device ID?
http://android-developers.blogspot.ca/2011/03/identifying-app-installations.html
In this video from last year Google I/O, Reto Meier talked about exactly that problem off identifieng a device correctly.
For the solution you will have to watch the video, because I don't remember it anymore. But I do remember his concerns about the "MacAddress"-Solution. The problem about this solution his that it addresses a particular device and not a particular "installation", think of the problem when a user, who installed your app, sells his phone to another person which will also install your app. With the MacAddress you have no chance to distinct between those two ...