Rails, although not the latest cool kid, is still quite relevant and a great choice for a CMS kind of site. Its undoubtedly one of the fastest ways to build web apps.
However there's a strong case for using a client-side framework like Angular, Ember or Flux, if you want to make your app super interactive. And thankfully there's tons of support (Angular, Ember ) for doing that in rails. One of the great things about rails is that it has a community which is very strong and rails somehow always brings things to table that helps it stay relevant. Like the new Rails 5 brings in ActionCable, which is a framework for real-time communication over web-sockets , inbuilt. It also integrates rails-api into core Rails project. Which I think is inspired by the fact that most people now use rails to build apis to serve more than one client. Its good to see how rails has acknowledged and serves the new trends in web development (SPA).
Regarding the SO support, because Rails has stayed here for over 10 year and has gone through tons of changes, lot of answers are outdated but I still believe if you ask questions on rails 5, there will be a lot of people who will be willing to help. And CMS is really one of the finer cases to use rails.