I'm potentially willing to pay a subscription fee for the right service. However, since the cloud functions are useless to me - I just want something to read on a single device; I'm content to manage file transfers myself - I consider a subscription a strong negative and the rest of the features would need to be exactly what I want.
Most important: Does it work offline?
I need an ereader app that works when I don't have internet access. Everything else is secondary. Dropbox allows me to set files and folders to "available offline." I can read books through it when I'm not connected. (Dropbox has a whole lot of other functions I find useful, like the ability to share folders and edit things after switching between laptops. I don't mind paying for features, but they have to be features I'll use. "Cloud access to things I can read" is not one of them.)
I want to know a lot more about it first. I asked some questions
in the BookFusion thread. I'm especially concerned with personal privacy and data harvesting. (Why do you need birthdates of readers?)
I couldn't find a section about GDPR compliance. I'm not in Europe, but a lot of members of Mobileread are.