I know these things exist, but I don't use them much even as an Amazon Prime member. Why? Because Prime Video UX is poor compared to Netflix (and Netflix is no great shakes either, but it's better than Amazon), and Music has a pretty poor catalog and again a pretty poor UX.
And this is mostly because Netflix wants to hide the true extent of their library.
Hulu just had a recent, awful re-design too. They cover in-active thumbnails with gradients, you can barely see anything.
Apple TV (which I like as among the best) also has a ridiculously unergonomic remote and very confusing UI (do I want to press "menu" or "home"? And now the TV app is 'home' by default, it's even weirder...somehow, even though I know I think how it works, I always seem to press the wrong one.) And no picture-in-picture either, though their processors are plenty fast enough to support it. But their standard built-in TVML-based interfaces are functional, if a little boring, and the UI is responsive. And hey, at least they aren't showing me ads in the UI or tracking what I watch. But still, lots of little problems.
So much room for improvement here, by everyone.