I think the problems people have with Gutenberg can be split into two main issues:

1) The content editing experience. As noted by many, as it is now Gutenberg makes content editing more difficult, slower and unpleasant experience. If you're typing and creating content you don't want to pause and have to click on several buttons for every line of text. Or be distracted by UI elements popping up all over the place.
This issue can be improved, as shown by the useful testing done by Millie and Anna. So it's great to see some potential improvement there.

2) The changes to the edit page itself. What is going to happen to custom meta boxes? What about theme code and plugin code related to the edit page, often custom written for clients, which is probably going to break many sites if Gutenberg is added to WP? For example, many themes and plugins create custom meta boxes for meta info, think about WooCommerce or custom fields created for a real estate website. Wordpress developers need to be able to fully create a custom interface to input that content. Meta info should stay out of the Gutenberg editor. Meta info and custom meta fields don’t belong in the main content editor. They also don’t fit in the small sidebar to the right.

So if the content editing experience of Gutenberg can be improved AND a good solution can be found for meta boxes etc, it might actually work out in the end.