Ruby on Rails Convention over Configuration (CoC) Issues to scale Large Rails Project

Working with Ruby on Rails for somewhile now. I'm enjoying it's builtin "Convention over Configuration" for small project. But, I'm little anxious about CoC application in Large Rails Projects.

I know some pre built-in configuration raises more difficulty than ease, when need to scale the application in broad.

What is those all specific Configurations will raise problem for me to scale? I will very grateful, if you please briefly explain those issues.