When the rich text editor went live (a year or two ago?) I gave it a try. In the middle of writing my first Rich research note I couldn't figure something out so I switched to the legacy editor which I was more familiar with. That click vanished the note I was working on which did not seem very fair. I don't know much about GUIs, but that link "For the legacy editor, click here" should definitely not have been at the top of the page for all this time if it deletes your work whenever you click it. So from then on I just used the legacy editor for all my notes.
Seeing the trouble folks have had recently posting notes, I tried the rich text editor tonight when I posted a research note. It worked fine for lots of text entry, although I stayed in "Markdown" mode a lot of the time.
After an hour of composing a note, when I was almost finished, the editor froze. I was in Rich mode, but the button to switch to Markdown said Rich, so I couldn't switch modes. The Publish button was grayed and unresponsive. There were no other options on the editor page.
So I clicked the Back arrow in Chrome which took me to a new, blank Post Note page. My note was not there. Nothing was there. But then I clicked the forward arrow in Chrome and was returned to my note and everything was back to normal, except for a few recent edits, and the title, and lead image, and tags I had added were missing.
I replaced the missing elements and then added a YouTube video embed to the note, but it did not display in Rich mode, and the iframe code disappeared from Markdown mode. I tried a couple of times, but there did not seem to be any way to add a video embed. And some other little errors had crept in which I could not fix in either mode. So I started over in the legacy editor. I had been saving the markdown text all along, so I started from a fresh Post page (legacy editor) and pasted the markdown text in and finished the note without incident as has been my experience since I swore off the Rich text editor.
I don't have any plans to try the Rich text editor again.