I'm a daily user on an android phone, as it's the only FOSS eBook reader still being maintained that has a feature set that includes white text on black background. It's pretty good, though it has some bugs, every so often an update breaks things and a new update fixing them is released soon, so I avoid updating if my version works fine for me. And since it's aimed at e ink readers it's UI is a little clunky for a regular mobile device. Great reader, renders everything you can think of, although every now and then with specific books it can get a little buggy. I haven't looked into the bugs to file reports.
I've used this on a very old kindle. I had a kindle voyage from 2014 and installed this, it gave my kindle new life I didn't know was possible. It was fast and it had all the modern software features I would want.
Suddenly I found myself being able to organise my library how I wanted (through the use of folders, like any directory). Very easy to manage.
Also it made it super easy to send books without having to rely on the cable.
Ultimately, it was the hardware of the device that made me look for an alternative. I just couldn't stand micro USB charging in 2023. So I bought a Kobo and later on a palma (for the form factor).
Being able to install Koreader has now become a feature I select for. Also, it's very easy to sync books in Koreader, if your device is always online you can do it automatically or the way I do it is with some swipe gestures to push and pull the progress.