Rails has plenty of critics. You just aren't looking hard enough.

The most common criticisms are: it's slow and it has too much "magic."

At the end of the day, it comes down to this: every framework has its pros and cons. When you use a framework, you gain certain benefits but sacrifice certain things and have to conform to the framework's opinions. In the case of Rails, the primary benefit is extremely fast prototyping. But that's only possible because the framework hides a lot of complexity from you, which can bite you in the ass if you try to go outside its conventions.