What you will loose is perhaps international characters. Unless of
course care is taken to convert from Emacs representation to
whatever w32 uses (UTF8?).
I think that has been done with menu strings (which is probably what I
confused this with) on several platforms already, and it might also be
possible for tooltips. However, I don't know how hard such a
conversion would turn out, and I don't know whether there exist any
serious applications that indeed require the interpretation of text
properties for tooltips.