Lua can be built on any platform with a ANSI C compiler.
Other than that, Lua is extremely small. For example, the tarball for Lua 5.2.1 is only 245K compressed and 960K uncompressed (including documentation).
When built on Linux, the Lua interpreter built with the standard libraries takes 182K and the Lua library takes 243K.
The small size and the ability to build with a C compiler make Lua an extremely portable language that can run on a lot of different systems and computers.
Pro Very easy to integrate with C and C++
One of the best features of Lua is its very well designed C API. This is very useful if you have an existing C library you need to integrate with Lua or quickly get a Lua script running on the C side of the game.
Finally Lua plays so nice with C that if you need to optimise for speed you can re-write it in C a lot easier than other languages.
Con Easy to make mistakes when declaring variables
When writing a function, if a programmer forgets to declare a variable, that variable will be declared at global scope. The code will seem to run fine at first, but if another function uses a variable with the same name, but fails to declare it, it will create subtle, incredibly difficult to find bugs.