calibre is multi-user (use the content server which allows multiple users to modify data concurrently) and does use a relational database, and all characters used in filenames in the library are safe on all filesystems (the only limitation being that windows cannot handle long filenames which calibre does generate on non-windows systems).
The only thing that is not multi-user safe is exporting the calibre library folder and running multiple copies of calibre on it on different computers, i.e. the calibre GUI is not multi-user safe in that you cannot run multiple instances of the GUI on the same library at the same time.
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