In reply to Mike Ellis.

Block patterns are only ever going to look as well as the active theme's block styles. If the theme is not well-rounded enough to handle blocks in a variety of configurations, patterns will quickly fall apart. If blocks are flowing into the sidebar and breaking the main layout, it sounds like a theme issue.

Spacing will definitely be different from theme to theme. That's to be expected.

As for making sure clients don't just insert any ol' pattern, I would just disable the core block patterns via:

remove_theme_support( 'core-block-patterns' );