For those who just want the meat: Gutenberg breaks WordPress. Not “official” WordPress, but WordPress as we have all known it and any code that has been written in the past.
For now, you can still disable Gutenberg, but the roadmap is to EOL the “classic” stuff by 2022.
So, basically—it’s time to give up on WordPress. Back to Drupal with me? Possibly.
And I suggest that small developers or small team developers everywhere move on. WordPress is dead. No, not yet, but given the security history of WordPress and the necessity of staying current with core, you don’t want to wait until official EOL for classic. Tell your clients and your boss now that anything you have built is EOL soon, will stop being maintained by you sometime before 2022, and you will be moving on.
Then, use the time between now and 2022 to develop a remediation strategy, build new skills, and move on.
December 2018 is the month that WordPress died.