• most formatting is based on style attribute which destroys website presentation style consistency
• there is no easy way to customize editor toolbar and inspector
• it is impossible to add more than one class to a custom toolbar button

Now think how all of those hardcoded colors and font sizes in pages content match up with accessibility when you allow visitors to switch to alternative hi-contrast stylesheet or to resize website font with a button. And what about implementing inline Bootstrap button link that needs at least two classes? Forget about it.

You can easily avoid all of the problems above with TinyMCE but with Gutenberg it is now impossible by the design.

Every WYSWIG editor that uses anything else for formatting than predefined tags and class attributes is not modern but obsolete because it is based on bad coding paradigms that should be avoided to keep website Kosher.