fontspec manual, there is a section on LuaTeX that states:
babelpackage is not really supported! Especially Vietnamese, Greek, and Hebrew at least might not work correctly, as far as I can tell. There’s a better chance with Cyrillic and Latin-based languages, however—
fontspecensures at least that fonts should load correctly, but hyphenation and other matters aren’t guaranteed. Under XeTeX, the
polyglossiapackage is recommended instead as a modern replacement for
What are the technical implications of this? Should I be afraid of anything?