I read that some of the key information is in attributes unique to Apple's file systems, I can't find the source but that was my conclusion after trying to figure out what went wrong in the Time-Machine/Synology/Rsync-via-webdav system I set up for my father some years ago.
If you rsync ext4 files to exfat, you also have issues, but those are very clearly reported when you attempt to do so.
Extended attributes are messy. E.g. the equivalent on Windows (ADS, not EA, which also exist separately) can store arbitrary amounts of data, I'd expect resource forks on macOS to be similar. Meanwhile Linux only supports 64 KB of data for them, though most Linux-native file systems have an even lower one block limit. And then there's Solaris where extended attributes are actually an FS namespace.
So even if both file systems support extended attributes, does not mean that you can actually preserve them.
This is the advantage of the "newfangled" backup tools which don't just copy files from A to B and can do (but not always do!) a much better job at a) not caring about your backup destination's file system b) not losing data that's more complicated than "name and contents".
