I've done twoa startups (2005-2006 & 2008-2014) with Rails (Rails <1.0-1.X && Rails 2.x-3.x) and we are now rewriting most of the software at my current company with Rails (5+) + React (for most things). We even went so far as to create our own framework with react, loosely based on how Rails does stuff called Gluestick (github gluestick). It lets us do native, web and mobile web with like 95% code reuse if you are good with web views.

When we were confronting the task of rewriting a large, undocumented, untested, msft db, and java (macro services) Rails was the easy part. Choosing a full fledged react app, with Rails as the api server was a harder decision, but based on who we had and what our immediate plans were it made sense. There is a decent argument now that just doing it all in Rails would of been faster, but the mobile part is going to be huge for us.

Finding senior rails devs is harder than it used to be though, hands-down. We converted a lot of django devs to rails and it's been a big win, according to them. "whalesalad" listed a lot of reasons why and I agree with all of those. We, at least in the past, at startups found Rails people to be more capable of full stack development and we needed that. But, that was probably just our experience in the 2009-2010 in the valley.