I’m using Publii now for a couple of weeks and I would say that it’s perfectly possible to create a custom website with it! It’s not so much different than any other CMS. They all use a template language to bind the user content with the HTML/CSS/JS code. I guess you can create all kind of websites with it! It’s just normal HTML/CSS/JS code! The only thing you can’t do with Publii (or any other static website generator) is processing a form or searching the website. But that’s it (I guess)!