Update: A more up-to-date version of this blog post is available at How to decouple Drupal in 2019.
There is a lot of hype around decoupled architectures, so before embarking on a project, it is important to make a balanced analysis. Your choice of architecture has implications on your budget, your team, time to launch, the flexibility for content creators, the ongoing maintenance of your website, and more. In this blog post, I'd like to share a flowchart that can help you decide when to use what technology.
This flowchart shows three things:
In practice, I believe many organizations want to use all of these content delivery options. In certain cases, you want to let your content management system render the experience so you can take full advantage of its features with minimal or no development effort (coupled architecture). But when you need to build a website that needs a much more interactive experience or that integrates with unique devices (i.e. on in-store touch screens), you should be able to use that same content management system's content API (decoupled architecture). Fortunately, Drupal allows you to use either. The beauty of choosing from the spectrum of fully decoupled Drupal, progressively decoupled Drupal, and coupled Drupal is that you can do what makes the most sense in each situation.
— Dries Buytaert