If there's something that really doesn't benefit from your model (reports), then you'd fallback to either SQL, or still use ActiveModel + the aggregates
What if you have a complex/dynamic query, how do you build it? You said yourself that AR works better than concatenating SQL strings, but AR doesn't even support CTEs atm and building complex queries is not trivial and sometimes even possible without just SQL strings...
With AR queries you can do the same and make it linear in ruby (and then the computer doesn't really care if your sql is nested)
last_three_posts = Post.limit(3).order(created_at: :desc)
@posts = Comment.where(post_id: last_three_posts)