Playing devils advocate here, Lua does indeed have some warts that make it less pleasant to use in a heterogenous environment (which pretty much all large code-bases are). Not so much due to deficiencies in Lua but due to the impedance-mismatch versus other languages.

The most obvious issue that everyone stumbles across is the "counting from 1". It seems like a minor thing, but the context-switch remains a drag when you're dealing with complex data-structures in two languages and only one of them is Lua.

The impedance-mismatch becomes even more apparent when the table-abstraction meets serialization. The lack of distinction between an "array" and a "hash" is awesome when you're in a pure Lua-environment, but it becomes a real problem when you need to exchange data with languages that do depend on this distinction (e.g. if you feed Lua an empty "array" it will later serialize it back to an empty "hash").

The final issue that I can't resist mentioning here is not a language but a community/mindset one. Up to this day Lua doesn't have an established package manager akin to RubyGems, Pip, Maven, Leiningen etc. (Luarocks exists but is... well, I've yet to see someone actually using it)

This is a deadly sin in terms of mainstream adoption. It makes deployment a serious pain in the ass.

GoLang shows how a modern language is supposed to handle this (importing/bundling packages directly from urls). I keep hoping someone will add something similar to lua-core, but I'm sadly not very optimistic about it.

I think many of the driving people behind Lua just don't care about it becoming a mainstream language or not. They care about it shining as an embedded language (and it does!) - it's just a little bitter for those of us who would love to use it on a broader scope.