At least on Windows Mercurial doesn't support filenames written in various languages. Git does. So Mercurial was made for one billion people while git for the rest 6.5 billion of us.
I don't know if that's still an issue.
As an aside, it'd be perfectly legitimate for Mercurial (and git) to reject filenames that aren't representable in unicode.
I was surprised because we've been using it for years at work, sharing repositories between Linux and Windows, and we have never been bitten by this. Maybe the latin-1 subset of Unicode works? Or all the files we worked on just happened to have ASCII names (despite us being mostly non-Americans).