polyglossia manual states that
Polyglossia is a package for facilitating multilingual typesetting with XeLaTeX and (at an early stage) LuaLaTeX.
However, I could not find any words about the consequences of its being "at an early stage". How much of it is supported in LuaLaTeX, and how stable is it? Should I feel "safe" when publishing a book using LuaLaTeX?
Note: The book I am going to publish is in Danish (but with some Greek text occasionally); however, for the sake of keeping the answers useful for everyone, I would suggest that people focus on the general question.