What is the implication of the warning in the fontspec manual with respect to babel and LuaTeX?

In the fontspec manual, there is a section on LuaTeX that states:

babelThe babel package 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—fontspec ensures at least that fonts should load correctly, but hyphenation and other matters aren’t guaranteed. Under XeTeX, the polyglossia package is recommended instead as a modern replacement for babel.

What are the technical implications of this? Should I be afraid of anything?

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What languages do you need? –  egreg Oct 6 '11 at 12:33
Currently I don't need any languages (except German), I just want to know what this means. What is broken? Why is it broken? I want to be able to give an answer if someone asks me: "Can I use fontspec with Vietnamese and if not, why not?" –  topskip Oct 6 '11 at 12:35
And you don't like the answer "Yes, but you should use polyglossia instead of babel"? –  Ant Oct 6 '11 at 12:47
@Ant no, because the why part is what I am interested in. :) –  topskip Oct 6 '11 at 12:49
@Ant: No, because polyglossia doesn't work (well) with LuaTeX. –  Martin Schröder Oct 7 '11 at 10:05

Languages that set an output encoding as part of their working in babel will have problems. For example, \selectlanguage{vietnamese} issues
\fontencoding{T5}\selectfont