Simply put the most advanced builder IMO on the market. They are improving their UX/UI to make it easier for beginners too. Being able to create any website without restriction appeals me and at the time of writing this they have a lifetime deal.
Even if you have a theme or builder missing a feature, Builderius can usually help without the need for the cost of extra plugins.
I would say, join their Facebook page and have a look at their features, if you like invest.
For somebody coming from FSE themes, Builderius definitely has a steep learning curve and its own logic feels different from what I am used to.
Instead of the usual block-based process where the designer starts with an idea inside their head and uses Gutenberg blocks to assemble a prototype then refine it, Builderius asks the user to create the components first and assemble them later.
This approach is perhaps overkill for simple websites, but it is ultimately much more powerful, especially for dynamic websites. And herein lies the ultimate value of this tool: whereas theme-based development tries to shoehorn the user into using WordPress for a particular task (a static site, a web shop, or something else), Builderius allows you to lean on the CMS capabilities of the WordPress platform to build whatever application you want.
I've watched this project for more than one year now, and I am periodically reviewing its development. As I am writing this, I don't think it is yet there in terms of functionality, and the community is still very small. But I am still convinced that buying an Agency licence was a good decision; I know I have at my side for a sophisticated tool that can help me for a project where a ready-made theme and block set would not be enough.
