Oh yeah the models are great at writing the DSLs, there are enough examples to do that very effectively. It's the building of the DSL, which is implemented in the config language, which is tricky. i.e, writing a new A/B test in the language is trivial, writing an A/B testing config DSL in the language is hard.

The main problem is the dynamic scoping (as opposed to lexical scoping like most languages), and the fact that lots of things are untyped and implicitly referenced.