@KhaledHosny I scrolled up and read your comments on how fonts are handled in XeTeX / LuaTeX; I agree and thanks for the work :-) If you'd like to test with some Indic scripts, I just posted some minimal examples at @UlrikeFischer's question:
tex.stackexchange.com/a/457459/48 (though these are trivial and no doubt HarfBuzz will handle them correctly... the point is just that LuaTeX doesn't)