I love block patterns. Love creating them. Love finding inspirational patterns whether they are actual WP patterns or not, and tweaking them.

This issue for me - and this is purely anecdotal, no hard evidence to back this up - is that your average user does not want to insert a pattern, only to then have to swap out all the content to make it their own. Building a full site this way is much more of a slog than say importing a demo site. Sure, with a full-site demo the content still needs manually changing, but the user has clicked once to import rather than clicking once multiple times to insert multiple patterns, and with a really well thought out demo there is a congruous design.

There is also the issue of - for want of a better word "flow". The stop-start nature of opening up the block pattern sidebar, searching for a pattern, inserting said pattern only to find it's not really what the user was looking for, or they don't then know exactly what to do with it.

All that being said, I'm really looking forward to how patterns will evolve, especially with block-based templates where the pattern content can be more dynamic.