In 2016, with systems regularly decomposing into services for both perf and code management purposes, I'm trying really hard to find a reason to say this isn't an invalid approach except for what amount to toy problems (the kind where "if you don't know where you're trying to go, any road will get you there"). ActiveRecord being a mudball of "what X is" and "how to get X" is a really, really big problem, and my view of it as an observer--heavy in Ruby, heavy in avoiding Rails at all costs--is that it seems to be sourced from an unwillingness of the Rails community to consider that not everything is Rails. Services become intractable, as I note in my reply to your first post--and I don't even mean "microservice all the things", just the profoundly unsexy and unbloggable "SOA".
Source: "The Rails Doctrine" http://rubyonrails.org/doctrine/
This is very much not Merb's design. But that doesn't make it an invalid design; it just means that understanding a Rails app might best start with the models, not the controllers.
(There's a lot not to like about Rails! It's been years since I worked in it full time and I don't miss it.)