I don't understand interest in dynamic languages. I have worked with them extensively, but I find them to be handicapped without benefit.
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.
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.