What’s not good for:
– large sites with lots of traffic (scalability issues)
– small sites with static or limited dynamic content
– APIs instead of full blown apps (it could be with some gem help, and it will probably be ok in rails 5)
– Fronting single page apps
What’s great for:
– Projects where constrain is time and money, since it packs so much functionality and let you develop fast and the most obvious security flaws you could be making are covered already