That brings up the other big problem with contemporary TeX. LuaTeX has handfuls of subtle incompatibilities with pdfTeX and XeTeX (and a weird way of handling character setting at the low level where it maps characters to their index in the font file rather than unicode code points). It's also noticeably slower than XeTeX which is slower than pdfTeX. It would be nice if it were true that pdfTeX ⊆ XeTeX ⊆ luaTeX, but unfortunately, it's not.