AFAIK, ADE/RMSDK based ebook readers cannot handle diacritics that logically follow a letter, but need to be displayed left of it. (For example, ADE cannot handle the
Devanagari vowel sign I and similar Tamil diacritics.)
You easily test this by embedding a font in an epub. For example, in my test file the sixth word in the first paragraph (bud'dhimāna) wasn't rendered correctly, because of the I vowel sign.