Just use Rails, it scales just fine. Your hand-coded DB layer for your microservice will never be as good as ActiveRecord if you aren't Facebook.
If I had to pick the top things that Rails excels at, I'd point to ActiveModel and resourceful routing. I know REST isn't the hotness any more, but it sure is great for CRUD web sites or APIs, and Rails makes setting up database models, associations, and REST routes a breeze.
One strong point is that it's full Ruby, so there is no need to learn another language. This means that one could write the application in the views (just like vanilla PHP) but in my experience nobody does it.
Another nice feature is that every instance variable of the controller is available in the view. No need to waste time to declare them and get an extra chance to add some bug (I'm looking at you, Django.) I still remember how good it felt coming from Java Struts in 2005.
If you send the wrong HTML tag, the content just might look a little funny to the end user. If you send the wrong datatype to an API client, it generally won't be able to do anything with it, and may not fail gracefully at that. Constructs to ensure you cannot make that mistake are invaluable in API design.
I wasn't looking for advice about web frameworks, but since you offered it, do you believe developers using other web frameworks are misguided in doing so? It's not like Rails is the only game in town, comparable solutions exist.