UX StackExchange tends towards more objective, fine-grained design issues (typically UI design issues) —if you see the list of recent questions, you'll see things like "pencil icon for create vs. edit."

Playbook positions itself as "actionable career advice," which is not at all what StackExchange provides in my experience. The questions are much more about process, workflows, advice.

So I think the title of this post is a bit of a misnomer. I've found both sites useful, but they're quite different in terms of subject matter.