The Apple TV's "TV app" is great for this... but for some reason, it only functions in the US. I get that it has a "live cable" Single-Sign-On component that cable providers need to sign up with–but all I want from it is the unified interface for searching/browsing and watch-tracking of content within streaming services!

I'd wager that's because (aside from Apple generally being US-based) international distribution rights are an even bigger minefield than US distribution rights.

It's alright but clearly dependent on the content providers and platform operators to be honest and accurate. Recently I tried to watch the new season of Archer. My Fire TV told me that show is available on Netflix and Hulu but didn't seem to know which seasons. I opened Netflix first and found they had old seasons but not the latest. I switched to Hulu and the thumbnail for the show mentioned Archer Dreamland, the latest season. I opened that up and found that the latest season was not available and Hulu had the same offering as Netflix.

FXX themselves do not seem interested in my money and require that I have cable so I continue not consuming their content.

The Roku does this as well, without giving preferential treatment to any particular vendor.