The language itself is very, very, very nice, don't get me wrong.

But the standard library is very small (deliberately), and while LuaRocks is fun, Perl, Ruby, and Python have a lot more libraries available. Plus, concurrency in Lua is ... nontrivial I loved concurrentlua, an attempt to implement Erlang-style message passing in Lua, with coroutines instead of processes. But it was kind of a pain to get it to work, and I think it was unmaintained.

When using Lua as a standalone language, this situation bothers me enough to stick with Perl or Python. However, when using it as an embedded scripting language, it is not a problem at all, because you will probably supply whatever library/API you want to be scriptable yourself. At that point you only care about the language itself and the API of its runtime, which is - IMHO! - where Lua really shines.

Well, it is true that the Lua ecosystem is small, but I've been following it for years and it has been in a steady growth. Yea, it does depend on which problem you're trying to solve. I'm using it on Machine Learning now and there are many libraries available on LuaRocks for that.