It makes coming up to speed on an existing codebase a slog because you have to trace through everything back to its source. Oh, and because there are magic methods and properties galore, your normal introspection tools in e.g. RubyMine get frequently stymied.

If you make a change to the return types of a function for example you have to manually find all of the different references to that function and fix the code to handle the change. Since there are no compile time errors it's hard to know that you got everything and haven't just caused a bug.