Readability is #1 priority
That’s arguable. Calibre is a database manager, not a reader. It has a reader, sure. But it’s an afterthought when compared to the rest of the program. The program is primarily aimed at people who have a reader and want to be able to manage their library. It’s days ahead of literally any other program when it comes to things like metadata management or managing multiple devices.
It’s sort of like saying that Notepad++ is bad at making Word documents. Like sure, it may be able to edit Word docs, but that isn’t what it’s primarily designed for.