I use this transform chain for asciifying filenames (ridiculous as it is, in 2019 we still can't sync unicode filenames between different OSs):

    uconv -x ':: Any-Latin; :: Latin-ASCII; [:^ASCII:] > \_'

Explanation:

    Any-Latin

Transliterates from non-latin scripts to latin script (e.g. "γραφὴν" --> "graphḕn").

    Latin-ASCII

Tries to asciify characters as much as possible by discarding accents, splitting ligatures, replacing Unicode quotes with regular quotes, © --> (C), etc.

    [:^ASCII:] > \_

Replaces any remaining non-ASCII characters with an underscore.