The key is that you're dealing with the shader language by using a language, rather than strings. You can use static type information. You can refactor more easily. You can create abstractions over the minor variations in card behaviors more easily.
While hxsl doesn't directly address debugging, it greatly improves your ability to create code examples to isolate the problem.
hxsl does about as much as you could hope for with shader languages, without requiring changes to the cards themselves (e.g. using byte code, which hasn't been standardized yet, and I wouldn't hold my breath for).
In some sense, the arrangement between app logic and shader logic is still a bit like client-server serialization. You still have to pass the data and the commands in a payload, since the two platforms typically do not share memory or have useful common architecture.
