I actually prefer the ATV app, but I typically use the 4K version on the TV like you do. The thing is, compared to other players, the ATV has more unique programming. My Samsung has VUDU, netflix, Amazon prime, YouTube, and other stuff I was using on the Roku. The Appletv has Apple Music, Home Sharing, Photos, the podcast app, AirPlay, etc. As a result, I prefer it to other streamers on my 4K TV.
The Roku is mainly being used with the Vizio 4K TV in my office because I prefer it to Vizio Casting. ( I use a splitter so I have the option to use it in either room)
ATV definitely has its merits, no argument there. Things I've noticed though with modern TV's, I'm using my KS9000 as the example, because that's all I really have to compare.
The built in decoder is much more powerful than the ATV4. Home sharing is great but has its drawbacks. I can pull encoded 4K/HDR videos directly off my NAS (without the help of my Mac) to the TV. I can encode a higher qualities too without the limits of the ATV affecting me.
AirPlay doesn't hold a candle to Samsungs casting feature. It's ultra fast and much more reliable, plus supports casting 4K. Apps work much better with it, you can queue videos, reliably move them into the background, etc etc. All my apps on my iPad from HBOgo to Crunchyroll support it...YouTube, Hulu, netflixs all those. There is no native mirroring though so that could be a problem for many.
All the sharing function can be performed without the ATV but like you I still prefer the ATV when it comes to photos. Samsung obviously doesn't offer direct access to iCloud photos.
Obviously Apple Music isn't built into a TV either.
Built in apps are better. I use YouTube as my example because I use it so often. There is still no refresh feature in the ATV app. To refresh my subscriptions I need to force close and reopen the app. This likely isn't Apples fault but if I just compare what is a better experience it's Samsungs built in app. Hulu is similar, slow and clunky on the ATV. Other apps can be very similar in quality like Netflix but again ATV doesn't offer 4K and HDR.
I'm finding other techs to be making the ATV more and more redundant. Why would I want to watch a lower quality video? Because the UI to get to it is slightly better?
I'll likely always have an ATV since I'm in Apples ecosystem. But with the upcoming PlayStation Neo the ATV will likely be my secondary device that I only use when I want to use iTunes or something.