Rails, if done correctly, is a remarkable tool to allow developers to focus on the business logic and UI of their app and forget about the "filler features" that pretty much every app needs implemented. On the other hand if you write Rails code without reading the docs and re-invent the wheel rather than doing things the Rails way, essentially running an app in the framework without using the framework methods, you can create yourself a monstrous unmaintainable codebase like no other.