I've had the opposite experience with the SPA projects I've worked on, one of them pretty large.
With the possible exception of authorization/authentication, the API for all my projects has been pretty straightforward, whereas the client-side app has been where I've missed stronger typing and rich data types most acutely, to the point where I've been actively evaluating compile-to-js alternatives with a better/safer type system.
However, with the advent of Facebook's Flow, I might just stick with JS.