Lack of advanced features like generics and a weak type system (although still better than JavaScripts type system). Its library ecosystem is also pretty anemic because it’s generally assumed if you’re writing Lua it’s as part of a script embedded in something else and most of those won’t let you pull in random Lua libraries. There are stand alone Lua interpreters that let you run Lua on its own but most people writing Lua code aren’t using them and so the community to develop a rich library ecosystem isn’t really there like it is in most other languages.