advantages of APFS library with FCP

You must never use NTFS or ExFAT drives for your libraries. They lack the appropriate metadata capabilities that FCP relies on, and sooner or later you will end up with costly problems.

It is widely recommended to use APFS for SSD, and Mac OS Extended (*) for rotating hard drives when used with FCP.

Here is an Apple support document:

Format storage devices for Final Cut Pro, Motion, and Compressor - Apple Support

(*) The linked support document mentions only APFS

What Luis said is absolutely right. Another advantage of APFS is file cloning, which happens at a block level.

E.g, in Finder if you right-click and duplicate a library or media file, with APFS it doesn't take any additional space, regardless of what Get Info shows on the new file or library. The filesystem presents the illusion of two separate files/libraries, but they are "backed" by a single physical copy. If one file is edited, only the changed data blocks are duplicated. This improves performance and reduces the number of erase-write cycles on SSDs. The cloning feature only exists within a single APFS volume. If you copy a file or directory between volumes, that requires a full data copy. For more info Google "APFS: Files and clones".

When an application hang/crash, sudden OS shutdown or power failure happens, ExFAT has no protection from that, HFS+ is better and APFS is the best. However, I would not use APFS on a mechanical drive due to various technical factors.

Ideally, you'd like all media, library, and cache on an APFS-formatted SSD, but for large amounts of data, that could be expensive. Lacking that, I'd suggest an AFPS-formatted SSD for a "lean" library (ie, using "leave files in place" and render cache outside the library), then the media files on a 7200 rpm mechanical RAID. If space permits put the render cache on SSD also.

It's also possible to leave the render cache inside the library (on SSD) and if you need to copy or move that, delete the render cache first. However, optical flow analysis files are not deleted using FCP menu commands. To avoid opening the library in Finder and deleting those, if cache files are placed external to the library via the Library Inspector>Modify Settings>Cache, this puts both analysis files and render files in a single location which can be safely deleted.

Most of our cameras record in ExFAT but that is write-once, not doing transactions. It's also best to not delete files in-camera, rather offload everything to an SSD hard drive (ideally APFS), delete what you don't need then reformat the data card in the camera. ExFAT has no capability to handle fragmentation, so if in-camera file deletes are done, it can sometimes hit a performance limit when recording subsequent clips and drop frames (or worse). For the same reason, don't fill the camera card to 100%.