Well, it would probably be 2-3 layers deep in a concern, really. Concerns are just fancy modules, and modules are just reusable parts of a class. In Rails basically everything is a concern. To a concerning degree actually, as a professional Rails developer of many years, I think they went way overboard with it. My team and I also curse it's single minded design when we have to drill down there. Thankfully that's a pretty rare thing to have to do.

As for the abstraction, Rails does take it pretty far, but you also have to take into account that it's a framework that supports numerous pluggable strategies and disparate backends, and extensibility. I think most mature frameworks are pretty complex-- the goal of the framework is that it's all encapsulated away from you for the vast majority of tasks, and in Rails, that is extremely true.