I have not found anything amiss at all apart from, as I have mentioned, some Store reading apps do not snap properly and share the screen with it. However, they are not ones that I would choose to use myself, plus I suspect the problem may lie with them given that MS's own PDF reader and Freda (and some others) work perfectly.
Regarding the low rating by the one respondent I would suspect that they just did know what Calibre was and were expecting an app with its own library (in which case they would have been stymied from the start trying to open a database). As far as I have found the app works exactly as you claim it does. Or maybe one of the strange denizens of web reviews who just cruise around getting their weird kicks by giving negative reviews to everything just for the sake of it.
When I get home at the weekend I'll relocate a PC to the USA
and give the app a good rating in the Store.
John