My main complaint is that there isn’t a consistent experience between apps. Things are in different places, actions do different things in different apps, no gesture support, subpar players with horrible scrubbing, etc.
I think all apps should be held to a few basic UI/UX standards so going from one app to another isn’t a completely different experience.
The attached photo is just one example. Within the Apple TV app itself, long pressing an icon brings up this great menu allowing you to go straight to the show or the episode page. Long pressing in most other apps removes the show from your now playing list! All of these conflicting actions need to be rectified.
If a developer is going to use their own player, they should be required to do two things.
Provide a reason which must be approved by Apple during certification. What do they want/need their player to do that the default player cannot do. There are plenty of good justifications for implementing a custom player, and the developer better have at least one otherwise what's the point? For example, the YouTube player allows users to thumb-up/down a video which is a thing they should expect to be able to do while watching YouTube via AppleTV.
Alternative players MUST be required to have feature parity with the default player. It's fine if they want to add more features, but they sure as hell should not be allowed to submit an app with a player that can't do everything the default player can. I want to pull my hair out every time I use apps that don't have the 10-sec skip feature, or ones that don't let you "circle the ring" to scrub, or even worse ones that don't allow you to effectively scrub at all.