I've been professionally working on SPAs for the past 7 years. I'm familiar. I strongly disagree with the premise that it's in any way more productive. I'm, at a minimum, 2x more productive building something the traditional "Rails way" with server-rendered HTML, and a handful of JS enhancements where necessary vs doing a full-blown SPA. I'd love to do a showdown, and challenge anyone to a head-to-head non-trivial app build-off to put these competing notions to the test.