I'm not an ebook reader, but I would have assumed that these apps would have some sort of indication if you've read a book or not and if you've not read to the end some sort of progress. Like opening an ebook that you did not complete should hopefully take you to where you left off at a minimum. I'd also expect your app to have a management type of display where I'd expect some sort of sorting/filtering where you can see only the books completely read, the books started but not finished, and books not yet started. I'd even somewhat expect a skeuomorphic layout of books on a shelf that you could somehow rearrange like it was iPhone 1.0. Again, I'm not an ebook person and never used any of the apps, so maybe these are standard things. However, it should make things easier to know if you've read them or not.

If I use the web interface to my self-hosted library, each book's cover is shown along with a progress bar if it has ever been opened in the web interface.

If I use the OPDS interface, that doesn't happen; I suppose it would be nice to push some reading information back. Sync between reading devices is handled by koreader-sync, so I can pick up any device running koreader and be on the page where I left off.

They do, but a physical book has a presence that digital books lack. Like weight, cover material, print quality. I can learn from digital books fine and read novels on an ereader. But a physical book anchor your memory like no other.