I have personally witnessed "legacy" rails apps (were ~4 years old, Rails 5 was around but we were on Rails 3 IIRC) and they are not really unlike normal legacy apps but they definitely let a lot of crap seep in due to loads of contributors and almost zero ownership on the codebase.
I wouldn't say this an exclusively rails problem though. Testing was a complete nightmare, and there was so much random middlewares that were "mission critical" but no one knew what they did. And as you said, scaling was horrible. We had like a hundred servers, each running 20 unicorn instances, to serve our API at the scale we had (~5 million users or so, I forget the DAU)
I wouldn't say this an exclusively rails problem though. Testing was a complete nightmare, and there was so much random middlewares that were "mission critical" but no one knew what they did. And as you said, scaling was horrible. We had like a hundred servers, each running 20 unicorn instances, to serve our API at the scale we had (~5 million users or so, I forget the DAU)