hi Martin – I work as the product manager for the team at the Foundation [i] that is responsible for building and maintaining the visual editor.
You can expect answers to the two questions you posed before Wednesday is over (PST)...I'm wanting to share the most up-to-date and exhaustive answers which require getting input from others people here.
In the meantime, two questions for you:
1. Are there particular wikis you had in mind when you asked this question? 2. By "Beta" are you referring specifically to projects where the visual editor exists as a feature within Beta Features [ii]? Were you referring to projects where the visual editor is not as widely available as you think it could/should be? Something else?
--- i. https://www.mediawiki.org/wiki/Editing_team ii. https://www.mediawiki.org/wiki/Extension:BetaFeatures