I don't know if I would call Rails templating excellent, but it is acceptable. The nature of HTML allows for quite a lot of flexibility. However, I have to disagree with your view of Rails in API usage. Maintaining a client contract for fields and value types is far more difficult in Rails, at least out of the box, than similar offerings.

If you send the wrong HTML tag, the content just might look a little funny to the end user. If you send the wrong datatype to an API client, it generally won't be able to do anything with it, and may not fail gracefully at that. Constructs to ensure you cannot make that mistake are invaluable in API design.