As we've seen recently with both the updated user profile and homepage designs, it's difficult sometimes to gauge the impact that this will have on users' experience just from screenshots or a video of the new UI. You can't accurately judge how this will affect different groups of users, such as moderators, who have different priorities and goals in viewing content.
While the beta-testing release of the new UI to the Greatest Hits page was a step in the right direction, it still fell short in that it wasn't possible to test how this would affect daily workflows, because... an unlinked, practically unused page isn't the same as the home page for your site.
In the past, when the UI was being changed, SE offered the option to opt-in to testing the new themes. Could we please bring this back for any major UI change? This would allow different user groups to provide feedback on how this change affects their use of the site, and allow SE to take that into account before releasing the change to everybody.