This article to me also implies that there are some potential issues with large Spec based development flows, which I haven't been able to pin down lately.

Specifically, having agent implementation drift from the Spec.

Drift is huge between any large spec and agent implementations.

I've done a ton of testing and the model doesn't matter, fable or sol still miss a ton of detail and drift.

I'm building http://engine.build which closes the gap and makes sure the implementation matches the spec.

It's not the same as the satisfaction you get when solving complex problems with code yourself but writing clear specs and thinking through the problem is still very satisfying to me.