After experimenting a lot with block patterns, I came to the conclusion that these should go the same path as shortcodes- in companion plugins for specific themes or theme developers.

These patterns go into the same container as page/post content, which often have styling geared towards readability, not specifically design.
So a H2 heading in a design element can/may look different from a H2 in a post content.

There’s only so much you can do with just rearranging blocks without any theme-specific CSS.
I created a plugin called Better Block Patterns, in which I tried to create a set of patterns that would look custom enough, but work with any theme.
It’s a big task to handle, especially when the HTML and CSS markup of core blocks keeps changing.

Having said all this, I think custom block patterns is one of the most exciting features and tools of modern-day WordPress.