Having written a ton of networking code in Lua I can tell it has a lot of design defects that most other programming languages have avoided. Especially the deeply embedded one-based indexing and lack of a separate integer type (chars are strings, ints are floats) make it wearing to type any code that has to parse any binary data. Even bit manipulation requires going through the FFI. Considering that Lua is often touted as the optimal lightweight solution in small embedded systems, these features are must-haves.
The builtin string pattern matching library attempts to mimic regular expressions, but bites the user in the leg every time they attempt to use it for anything remotely regex-y. Unfortunately most usual string manipulation seems to revolve around them and the string.gsub function.
The inconvenient string library is also largely irrelevant. The developers call their language eight-bit transparent which in reality means that the programmer needs a separate Unicode library, rendering the whole standard string library mostly useless. One cannot even format UTF-8 text, because the string library doesn't differentiate between characters and bytes.
The official documentation mostly revolves around C bindings. The actual Lua language documentation is provided by another web site that looks like it has been written by an attention-deficit child. Instead of getting to the point, the articles often first provide a list of ways on how not to implement things, or they are actually benchmarks of fifteen different ways to call the standard library to get the same effect. Sometimes, the provided code does not even work – for example, the given XML parser code crashes when it hits one of the error paths.
The small runtime and fast VM do not really matter when the language itself is lacking and requires a metric ton of external libraries to achieve even the simplest things that are possible in standard C and Rust, which provide the same level of performance and better type safety.