As FireOS is based on Android, the apps (Netflix and Spotify) look and feel almost identical to the Android TV versions. I would think the same about the BBC iPlayer app, but I don’t use it myself.
But Amazon‘s new OS (Vega OS) is said to also be Linux based (like Android) and rely even more on web technologies. Maybe similar to LG‘s WebOS they use on their Smart TVs. So I don’t think app developers will have problems porting those apps over.
And that’s my point here: Apple doesn’t allow WebKit on tvOS. So developers can’t rely on those web technologies and use the same shared code base on Apple TV. But as a hardware device, Apple TV is just not important enough, so they will spend much time and money with developing a native tvOS app. So I think that’s why we end up with those apps that feel very basic compared to other devices.
But I agree that it doesn’t help that especially Netflix and Spotify aren’t really big friends of Apple. Of course both can’t afford to ignore Apple‘s phones and tablets. But I guess it doesn’t hurt them much to kind of ignore Apple TV.