I want to keep this as short as possible, so I'm going to skip for now many issues and bring up only the most important ones (for now)...

Most of us develop sites in a localhost environment. When we use FSE to create or edit template pages/parts, those designs do not get written into files, but get stored in the database. When done, and the fully customized theme is to be delivered to the client/live site, we just cannot package the theme files, we also have to export and then import the Templates, the Template parts & the Global Styles from the Tools menu - not a big deal for most, but many will have issues with this, as a simple Theme installation will not work in this case.
Patterns: They are great in theory, but because the whole Gutenberg project is not planned and built correctly, very naively if you ask me, there are some issues:

a. Lets get to the purpose of patterns - to save you dev. time. But because Gutenberg does not have universal padding/margin, and block basis custom CSS coding facilities, all we can get out of the box are weak, pathetic and embarrassing patterns. Putting 2 buttons next to each other, and serving that as a pattern does almost nothing in terms of time saving. This is causing 2 major issues: The first one is that many third party blocks provide their own margin/padding/animation/css controls, which makes the whole experience very chaotic and not uniform. For each block library you have to learn where everything is, as oppose to a one nice Core universal UI.

b. And then we have the Hansen theme from the theme repository. Very nicely done, which comes with some nice patterns. Because Gutenberg lacks all the features mentioned above, the theme uses some custom CSS classes, and these classes are coded in the theme's style sheet. The problem with this is that now that you have used these patterns, YOU ARE LOCKED IN to this theme. Because the moment you change themes, the new theme will not have these custom classes defined, the patterns will be broken. This is THE SAME reason why shortcodes were outlawed many years ago from inside the themes - and yet when it comes to patterns, this is somehow allowed??? Will custom blocks be allowed in themes next?

The site templating system does not allow third party blocks, we are limited to what they allow us to use, WHY?
The menu page is gone, that's semi ok, as we can do custom menus from the customizer, but the customizer screen can now only be accessed from the Themes page, with a button on the Theme preview image - WHY??? Now you can create custom menus, and tell the Navigation block to show it, but they are not connected. Once you add a new page/link in the custom menu, the new links don't show up in the nav block. At this point you have to add the link in the nav. block manually (again), or delete the Nav block, and start a new one, to get the current links from the custom menu.

I know that this whole thing is not done, so I can't be very critical, but we are a month away from confusing anyone who is not following the project, and many issues could have been avoided, it the foundations of this project were built correctly. I don't think there are any serious page builders that don't have padding/margin/css controls, except Gutenberg of course.

P.S. I also wonder what happened to "decisions, not options", as almost all decisions are taken away from the themes and are given to users, which will result in many ugly site designs, and confusion. Last thought, are boxed themes even possible with FSE? I can go for 3 more weeks ranting... at least !

My criticisms are to be used ONLY in a constructive manner...