Here's a measure of how fragmentation of Android versions and phones can be a challenge. Microsoft's Remote Desktop app has been downloaded by more than 5000 unique device types.
While that's an enormous number of potential variations, it's worth remembering that those are device types which successfully downloaded the app -- the Play Store won't let you install an app if your phone or tablet isn't compatible.
The variety of phone designs and the fact many users never upgrade from their original Android software version is often cited by developers as an argument to favour iOS over Android. The Microsoft example shows that it remains an issue, but also that it can be overcome with intelligent coding.