What do you think about mixing
luacode? Is this an "accepted approach" or rather a "bad practise" or something in between?
Consider, for example, my previous question Nested splitting of a sequence using expl3 and the excellent answers by egreg and Mico. I have started writing a package based on
expl3. In this specific case, however, the LuaTeX solution is much more concise. This is why I would tend to call a LuaTeX function from an expl3 function.