In many ways the problems people have with RoR are the same problems people have with ORMs. They abstract away a lot of the clutter, but then, just as you become totally dependent on the abstraction, you find you need something that you can't get in the abstracted layer.

When this thing is a substantial increase in speed, sometimes you are totally f*ed, since to get it you need to toss away a lot of the detritus. For an ORM you can gradually phase away parts of it, but for a web framework generally you can't architect away the box into which your app is placed.

I'm unaware of any RoR that has successfully scaled, except by removing Ruby (Twitter). Github is probably the largest existing Rails app (at least that I use), and there are Unicorns aplenty (the new fail whale), or weird caching issues that seem to arise regularly.