It's true. I doubt this spec will pick up even a little traction either since it's not backwards compatible.

You trade slightly more concrete data structures for hard incompatibility.

Maybe an internal api could leverage this in some way?

But it really isn't that hard to wrap hex, floating points, etc. in quotes and parse that server side.

Any application that's not a pet project should be using json schemas anyways, which is the layer you could put your custom datatypes into.