I was thinking that if you draft a document in XeLaTeX/LuaLaTeX, including a template, then if you don't bother to really learn how font handling with XeTeX/Lua

I was thinking that if you draft a document in XeLaTeX/LuaLaTeX, including a template, then if you don't bother to really learn how font handling with XeTeX/LuaTeX works, you might end up with some fontspec or polyglossia line in your code, etc., and there might be problems with a template that does things oldstyle. Another point is that some packages haven't been updated for Unicode, algorithms2e being one popular package that's still Latin-1, IIRC. Then if your code will be compiled from source by someone using pdfLaTeX, your Unicode characters will cause problems. Hence why I say that it's probably more pain than it's worth, since there are still things you need to understand to avoid incompatibility. I think a more proper analogy would be C and C++, since while XeTeX/LuaTeX is mostly LaTeX compatible, 1) you risk running into sinister incompatibilities and 2) typical XeTeX/LuaTeX code is different from, and incompatible with typical LaTeX code.