The number one complaint I see, and one I've personally experienced, is aggressive monetization that actually breaks the reading flow. It's not just ads—those are annoying but expected—but the paywalls that pop up right in the middle of a climactic chapter. You'll be deep in a scene, heart racing, and a full-screen overlay demands coins or a subscription to read the next paragraph. It feels predatory, like the app is engineered to exploit your emotional investment. It cheapens the whole experience and makes you feel less like a reader and more like a wallet.
A close second is clunky library management. I download a dozen samples, but the app sorts them alphabetically by some internal file name, not the actual title. Finding a specific book again is a chore. No proper series grouping, no way to create custom shelves beyond 'favorites,' and the search function within your own library is practically useless. For people who read a lot, a bad organizational system is a deal-breaker.
Finally, a weirdly common nitpick: terrible text rendering. The font choices are limited and ugly, or the justification creates massive, uneven gaps between words. You can't adjust margins or line spacing enough to make it comfortable. It sounds minor, but staring at poorly formatted text for hours gives me a headache. A good reading app should disappear, letting the words take center stage. When the app itself constantly reminds you of its presence through bad design, it's failing at its core job.