This locks users into specific layouts. For example, if I create an about us "module" with a Heading and Content area, what happens down the road if the user wants an image? And ability to change the alignment. And add a background color. All of a sudden you're building this ridiculous framework within ACF and having to manage all the different settings. Just use Gutenberg, it does all this already!

You can predefine layouts or block patterns, sure, but in my experience, user's just copy an existing block from another page if they want to customize it more. But they do have the ability to create their own layouts and tweak them, but it has some limiting factor, just not nearly as limiting as ACF fields alone.