Thank you for your comments!
After a period of expansive development resulting in a lot of new features, we are currently in an intensive bug fixing phase working hard to streamline the Runbox 7 interface.
We have organized this phase into several “quality” milestones over the next few weeks and months as seen here:
All the issues mentioned here in the forum should have corresponding issues on Github that we are working to resolve, but we would in particular be interested in more details about the UI being “sub par”.
We are of course aware of some aforementioned aspects relating to this, but the more opinions and discussions we can generate the better positioned we will be to make Runbox 7 the best email interface possible.
Runbox has never had a cookie-cutter approach to email and service development, as our goal since the very beginning has always been to innovate and find better solutions than the existing status quo.
Ready-made solutions such as Roundcube may appear attractive, but they would severely limit our offering and flexibility in developing exciting features and functionality. Additionally, a custom-built UI can be integrated with related services such as identities, calendaring, task management, file storage, email/domain hosting, settings, payment, and admin functionality, and a number of features we are planning for the future.
You can find more background for the Runbox 7 project via our Roadmap here in the forum.
We greatly appreciate your feedback and support, and can assure you that we are focusing our resources towards improving Runbox 7 to a level you will be satisfied and impressed with. 
– Geir