A lot of discussion I heard around writing idiomatic Rails led to really fat models. Given the number of database trips back and forth with ActiveRecord, it didn't end well.

Eventually as the system grew it was much better to have bulk-interfaces of data-only for reads that completely bypassed ActiveRecord. Logic in controllers was also eventually factored out and re-used elsewhere.

My experience is that a lot of advice for Rails centers around small-to-medium size applications, and of course practices that are efficient and practical for apps of that size might not work exceptionally well in other scenarios.