An important consideration is what benefit this brings to people other than the developers making it. In cases where the WP REST API and decoupling WordPress brings a real benefit for the client then that's great. Go with it. But too often the developers I've seen and talked to want to bring in AngularJS or another JS framework, decouple WordPress, and over-engineer a solution that results in the front end experience for users having no improvement yet introduces more complexity behind the scenes. The developer gets a nice piece for their portfolio, and likely gets to charge a premium for the project, while the client gets...to pay more and require more specialized developers in the future to give their users an experience no better than their competition who went a more traditional route.

Don't get me wrong. I'm not against the WP REST API or anything along those lines. It's a powerful tool, but use the right tool for the right job. I'm tired of seeing people using the newest, shiniest tool on every job whether it brings a benefit or not, just so they can brag about it, while other people invent newer, shinier tools to accomplish the same thing so they get to brag instead. But that's the state of front end development right now. Advancement, yes, but with lots of reinventing the wheel and selling clients a bill of goods.