It's gotten much much better in the modern times. Android 2.x would break your app in many spectacular ways, but Google has been steadily adding more coverage to its "compliance test suite" that a device must pass to be eligible for Play Services preinstall. On 4.x, there were some Chinese phones, like Xiaomi and Meizu, that meddled with notifications, action bars and list views, sometimes to the point of making your app crash, and I do remember having to work around them. But if you support Android 6.0+, like many new apps do these days, you don't have to worry much about device compatibility.