^This. I like Moon+ as the nicest and most robust android app, but practically speaking it's useless to me as I cannot sync it between my PC, android, and e-ink devices.

In addition to that, Google Play is the one place I can upload other non-DRM'd epub books and get the full functionality of their platform. It's the least hassle and way less hassle than trying to maintain a cloud library of my own to sync across multiple devices.

Anymore all I use Calibre for is to DeDRM, clean up and convert books when needed after original purchase. I convert to epub, upload the ones I'm currently reading to Google Play, and done.