Comfortable is described as 'cloud-based API-first CMS that can be integrated fast and easy with any framework or platform (or even with an old-school HTML Website)' and is a CMS Tool in the Business & Commerce category. There are more than 25 alternatives to Comfortable, not only websites but also apps for a variety of platforms, including Self-Hosted solutions, Linux, SaaS and Mac. The best alternative is WordPress, which is both free and Open Source. Other great sites and apps similar to Comfortable are Strapi (Freemium, Open Source), Cockpit (Free, Open Source), Contentful (Paid) and OverBlog (Freemium).
Squidex is an open source headless CMS and content management hub. In contrast to a traditional CMS Squidex provides a rich API with OData filter and Swagger definitions. It is up to you to build your UI on top of it. It can be website, a native app or just another server.
Typewriter is a headless, API-focused content management system that works with any platform you want to use. Create pages and collections of documents and combine them to build a powerful content provider for your API-driven website or app.