PHP frameworks are typically used to build a product/ application quickly against a Java or .NET stack. Headless Content Management Systems, such as Drupal 8, gives a paradigm shift to create and release Minimum Viable Product (MVP) quickly.
Headless CMS, e.g. Drupal 8, gives that robustness of using AngularJS/ EmberJS (and similar) to enhance the CX (end user experience/ customer experience).
With headless CMS the back-end and front-end teams can work separately. It also gives an ability to serve content on the growing numbers of websites, back-end systems, and different types of devices (e.g. wearables).
Before headless CMS the browser would show a themed output that the CMS generates. With the decoupled approach we handle CMS' job of creating and displaying HTML to the front-end framework like Angular.JS, React, or Ember.JS. The data exchange between different parts of the application is supported by RESTful API. It allows different parts of the application to communicate with each other.
With that said, decoupled CMS isn’t a magic bullet which enhances the performance of your site. Attention needs to be paid UX, UI localization, search engines' indexing problems. If your MVP needs HIPAA compliance, will recommend hosting it on Acquia.
We, at Faichi Solutions, have been creating applications and products using headless CMS. However, with Drupal 8 and its OOPS support, gives that extra edge to build Healthcare applications quickly. As we all know creating and releasing MVP allows startups to showcase the product and seek funding. Gone are the days when you could create a powerpoint presentation and seek Million dollar (or more) funding.
Look forward to your views and alternatives.