I have mentioned this in the plug-in reviews and have been testing each major version of Gutenberg.

1) Some of the haters are disabled folks. For better or worse, they have the force of law behind their complaints. So many educational and governmental institutions will be legally prohibited from using Gutenberg until some time in the distant future.

2) Gutenberg is requiring a rewrite of many plug-ins. Not all of us have the money to rewrite those plug-ins, and it will take time, anyway. Even if we could afford the rewrites, we don't dare do much rewrite much until we see what Gutenberg actually becomes, and whether we will have to pay for a major rewrite for each phase of Gutenberg.

3) The paragraph-as-a-block approach won't work well for many authors who write in great volume.

4) The unified tool bar is an improvement, but the interface is still dreadfully unintuitive for our users' needs. (And some of our power users have already threatened migrate to something else).

5) Gutenberg violates the fundamental web design principle of separation of design and presentation. When web pages don't work in some devices, users blame the platform (which means us), not their wonky overrides.