Lua is a fantastic language and the people behind it have great goals. To keep Lua fast. For the common developer like myself where speed actually doesn't matter and additional functionality would be nice. Luautil is a utility belt that adds that functionality. Luautil is based off of Underscore.js plus some more functions that other languages like Python and Javascript have that Lua doesn't.
Underscore.lua isn't a full implementation of Underscore.js and it doesn't add functions like "slice", which is present in almost all languages. Lua has valid reasons for not supporting slice but again performance isn't a goal. Also, if you look at the people behind lua they use certain conventions that should ideally be law.