What worries me (I write Django, Rails, Phoenix web apps - different customers at the same time) is that I'll have to learn yet another three different ways to do the same thing. What I'm doing now is either creating HTML pages (the 3 ways are pretty much the same, Django a little bit more set apart) or a JSON backend (basically the same everywhere.)

But to be fair it's fear of the unknown: I read about all of these approaches but the only new project started with a SPA and the other ones will never move from backend generated HTML.