Here are just a few notes:
There are quite a few design choices that makes Channels difficult to use with large libraries. Most of these observations are going to be specific to the Android clients (since that is the main interface in our household).
The main issue that makes large library navigation difficult is the choice to solely use a tile/card paradigm. While using large images makes for nice visuals, the inherent lack of information density makes it a poor choice conveying large quantities of information.
There also currently is not a good way to view through in-progress/currently watching programs/movies. While there is something like this for Apple clients, it has not made it to Android yet.
TV Shows
The image/thumbnail interface could be forgiven as the primary choice for selecting series, but carrying it over to the episode list makes no sense. The thumbnails used are just a screen-cap from a point early in the recording, and really has no real meaning to the episode itself. Also, the horizontally scrolling list means very few items can be displayed on the screen at once. The metadata is only selectively displayed in the list—either episode number or air date, but not both—and to view the full details the detail modal must be opened.
When viewing a program with many episodes—we have a few programs that we record and keep every episode—scrolling to the end or looking for a particular episode is quite a chore.
While Up Next saw recent improvements, it still is a bit lacking. The order in which items are added—or even if they are added—is still opaque, and a bit mysterious. Also, when browsing from the main Library view, episodes are titled with the season/episode number; however when viewing Up Next fullscreen, the episode's title is used instead. Also, for some programs that are inter-related (Arrowverse, Chicago, etc.), having the date/time available would help choose which program to watch next.
Movies
The viewing of movies is awkward, and inconsistent. When selecting a movie from the inital Library view, it is presented in the same detail modal that TV episodes use. However, after selecting More and viewing the full library, selecting a movie presents its information in a different style of view. This “Movie view” is also a poor use of space, as long descriptions are truncated about half-way down the screen, and no way to view the full description or scroll the text is given.
When viewing movies alphabetically, there is no way to quickly navigate to a particular place/letter. (Plex achieves this with an alphabet list on the right that can be navigated to and selected.)
While “enhanced” library options are slated to come to Android, they still aren't there. Having an option to browse by genre would be nice.