To answer your question, YES! Rails is a viable server side framework that is mature and well supported. It is used by literally THOUSANDS of companies internally and externally. In most instances, there will always be a need for a server side framework if you are working with Javascript frameworks on the client-side. Don't be seduced by "the latest and greatest" language or framework. Companies want PROVEN and STABLE technologies that will minimize issues for them.
Many of the "hyped" down-sides of rails are no longer issues and yes, MANY major players still use it, Shopify, Github, Hulu, AirBnB, Bloomberg, Fiverr, Groupon, and many many more....
Here is list of 37 top sites that use Rails:
skillcrush.com/2015/02/02/37-rails-sites
Also, Here is a great article to review and will give you some insights as to why Rails is still being used by so many companies.