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.

toptal.com/ruby-on-rails/after-two-decades-of-pro…

Shopify is the only Rails shop I know

The main reason that big companies stop using Rails is because they have A LOT of traffic, and is because other low-level languages are faster to serve requests than Rails.

However, you don't choose Rails because is faster than other frameworks. You choose it because it allows you to code things fast in a very elegant way (the keynote of DHH for the RailsConf of this year was related to this).

Eventually, if at some point in time you need to handle a lot of users (so many that Rails starts slow down), you will probably have money to handle that (i.e. buy/improve hardware, hire more programmers to improve your app, etc).

And finally, if you're worried about job opportunities, there are enough out there (maybe not much as javascript, java, or other technologies, but there are enough jobs :-))