After just short of 2 weeks with an ATV 4K, both myself and my wife agree it fall's short of the FTV on many fronts.
This is not a reflection on Channels itself in any way, as to be fair to the devs the app functions perfectly on the ATV 4K, there was no stuttering or image issues at all on it, until the FTV decoding issues are fixed the ATV video experience is superior. I do prefer the layout / I am more used to the layout on the FTV than the ATV but the actual viewing experience on the ATV is fantastic.
That's about where the good news stopped for both us of, the remote was awful to use to such an extend I bought one of these:https://www.oneforall.co.uk/universal-remotes/urc-7935-streamer-remote which improved the experience but had more buttons than I really required, felt cheap and was still far from the usability, comfort and quality of the FTV remote.
I also liked the in app info that the ATV homescreen brought you which worked well quickly jumping to a program in channels however many other apps did not implement that feature quite as well such as plex which just kept taking me back to a black screen after logging in. It would be nice to see this feature brought over to other streaming devices so allow you to navigate an app without actually launching it.
I really wanted to love the Apple TV, but overall my wife found it more difficult to navigate and less responsive, I think a much better designed remote would really benefit the ATV but even with the same remote I would still pick the FTV as it stands. Especially when FTV 4K's can be had for £25 quite often, yet the ATV 4K plus an additional remote is £210, ok for a single TV but I want one connected to every TV!
Now I just need Amazon to fix the decoding issues that seem to have gone very quiet! If they can get that sorted apart from the ridiculous amount of Amazon advertising I would recommend the FTV 4K as a client device.