Flexibility is available in Rails. But is not a top priority. I would say that Rails gets most of its flexibility from Ruby. You have Rails Metal, which is intended to provide a modular way to extend Rails. If you know Ruby well, you can take advantage. Solnic had the capability to do so, and did, but eventually chose to leave the ecosystem. I'm fine with that. I don't expect the techs i use and love to be all things to all people. I just wish he hasn't chosen to shit on the tech that sustained him for so long.