In reply to Edwin Cromley.

I have tested Gutenberg metaboxes support, and it does work with some plugins, but it is not working with a lot of plugins, ranging from JavaScript errors, styling issues, not saving data properly, to completely broken.

And, due to the use of IFRAME, pages are very slow loading (everything is loaded twice, for the main page and for the IFRAME), and there are many more issues. To make things worse, use of IFRAME causes problems with in-browser debugging.

Gutenberg is far from well-managed project. If it was managed properly, metaboxes and other compatibility issues would be discussed before the work has begun, and not after a year of development that all involved knew will be impossible to support metaboxes properly.

IFRAME is not a solution, and right now we are looking at a high percentage of broken websites.

There are so many things that WordPress development involves, and the editor as it is now, can't be considered a priority. And, holding off the WordPress development for the unspecified amount of time until Gutenberg is ready is just wrong. WordPress 5.0, 5.1 and maybe 5.2 should happen on schedule, and Gutenberg might be ready by the end of 2018 for merge. Holding of 5.0 for a whole year is a very bad solution.

And I don't see any way for Gutenberg getting merge ready before the end of 2018, considering it's current state and severe lack of testing.