Nailed it. Composite resources and good use of caching with fine-grained resources can go a long way.

HTTP is a pretty sophisticated protocol, when all is said and done. That means there is a learning curve in using it effectively. However, a good BaaS takes care of that for you, including providing default client libraries that know how to take advantage of the API properly.

(Full-disclosure: my company, spire.io, has recently launched a REST API BaaS that attempts to address exactly these concerns.)