> avoid [...] accented characters

It's certainly good advice and I definitely avoid using non-ASCII characters in filenames in practice. But I can't help thinking that advice like that is why support for Unicode is still buggy in many places.

I see nothing fundamentally wrong with using non-ASCII for filenames (and the slides don't give any reasoning), if only random software wouldn't mangle encodings, sorting order, or plain refuse to accept such filenames.