[jnbiche]

> Lack of purity.

I agree. Which is why I said "many if not most" benefits, not all benefits.

> It's impossible to validate that external JSON data is the correct type at runtime.

Not sure I understand what you're saying. Of course you'll get a runtime error if your external JSON is the wrong type, what else could you do? Accept the data and enter an inconsistent state?

However, similar to the Elm JSON validation, in Typescript and Flow, you can write schemas to to runtime validation of external JSON. There are multiple libraries for this.