The problem is that most dynamic languages (like JS) are never designed with speed and efficiency in mind. JS just happens to be the fastest of that bunch (other than Lua) because browser makers have invested substantial effort into making it fast - far more than anyone has put into other dynamic languages.
However, JS specifically was not designed at all to begin with, but just thrown together, and core concepts have stayed since this first iteration. These core concepts are its primary handicap.
But yes, in general the same lack of static guarantees that makes dynamic languages slow also makes code in dynamic languages hard to maintain. I think a lot of programmers tinker with code in dynamic languages and end up liking them as a result, but don't realize the difficulty of working in such a language in a much more complex codebase.
I love Lua personally, but it has its own traps too. Never could get into Python too much, the indentation syntax is killing me :P
I no longer see dynamic languages useful for more than bash-like scripting where correctness is uninteresting. :/