Every Rails project I've been involved with has had two phases:
The first phase was being amazed that we got basic functionality up and running so fast
The second phase was feeling like we were spinning our wheels because we were always dealing with some performance issue or some complexity explosion that occurred when we stepped outside of the bounds of a simple low volume CRUD app where Rails excels.
That does not mirror my experience. What I did see a lot was either spinning wheels because multiple people had divergent ideas about how the complexity should be managed, or spinning wheels because people on the project did not want to work with Rails at all and just wanted a rewrite of everything in something else.
Complexity does not have to be an explosion, but it always will be if the team designs in a way that is not coherent - and coherence is hard to achieve.