Heya. Full disclosure, I’m one of the 2 developers on Channels so my opinions may be biased
One big advantage of Channels DVR is that it doesn’t need to transcode the MPEG2 files when playing them. The Channels app plays them natively. That means changing channels or playing back recordings happen within a second, vs 5-10 seconds on Plex. Our customers like that a lot, along with the more traditional and simple UI. Our customers’ families like that the MOST. Plex can get a little confusing.
I personally run the Channels apps all over my house on Apple TVs. It really is the best streaming box. It’s more expensive, but it’s way more powerful, the UI is cleaner, and there’s no ads. The Apple TV app is pretty much our flagship app. It works really great on the Shield too.
It’s also important to know that only the tvOS and iOS versions have full support for DVR at the moment. The Android TV and Fire TV versions support browsing and playing back your recordings, but they don’t have all of the UI for managing things like creating season passes and stuff. That part is coming really soon though. For now you can just manage that with the web admin if you really want to go with the Shield.
As for your other question, because Apple TV apps can’t run in the background, they can’t be launched remotely. So if you were to use Magic Cards, the app would have to be open. The Android/Fire TV version can though, as long as the app has been opened once and is still running in the background.
I hope this answers some of your questions!