FTP (and friends) indeed are a factor as well, but what stuck in my mind was how Forth initially was supposed to be named "Fourth", but the OS didn't support file names this long. https://en.wikipedia.org/wiki/Forth_(programming_language)#H...

HFS (and HFS+, and APFS) indeed has "resource forks", and supporting (or just not tripping over) them on non-Apple filesystems comes with a bag of surprises, even on Apple platforms. Just last week we've run into a problem with a script that globbed for "*.mp4" files to feed them into ffmpeg, but ran into and choked on the "._*.mp4"'s (which is how macOS stores the resource forks on FAT32/exFAT). The script was tested and worked fine on every Mac I tested it on, and everything was smooth until the customer plugged in an external HD to process videos off of. https://en.wikipedia.org/wiki/Resource_fork

So unfortunately the most reliable way of knowing exactly what you're working with, is lots of heuristics, and very defensive coding.