Recently I learned that sorting all existing lessons (not just within a particular category) by the % of known words is only possible in the web version. And even that only by using a secret, non-intuitive trick. It is a pity, because finding texts that are within the Goldilock zone (with ca. 20% of unknown words) is a very important part of keeping one’s motivation high, so I don’t understand why such a fundamental yet easy to implement feature is not easily available in the app.
For me, the mobile app provides a smoother experience when studying lessons. Web/desktop/browser version feels (to me) a bit clunkier for this purpose, but provides access to more functionality. I use both.
I exclusively use the app for reading, on my iPad and iPhone. I have almost never had an issue with it.
I use the web for uploading content and organizing lessons. I have never tried to read from the web, preferring to curl up in an armchair with my mobile device.
I try to use the iOS app for reading but get frustrated by how much slower it is to add and change LingQs as compared to the web version. Mostly I use the iOS app to review vocab of the lessons that I’ve been doing as the web version doesn’t allow you to do this (you can’t only see some of the LinQs/New words on web version and you can’t change them).