The modern UI is intriguing. Unfortunately development by Microsoft has been extremely slow. The experience is handicapped by its lack of support for calendars. The problems with list functionality (column totals dont display properly etc.) mean that frequently our users have to switch back to Classic view in order to perform their tasks. These problems have been in existence for some time now. When will Microsoft understand that simply slapping a fresh coat of paint on something is not a solution to problems within their software.
At this point and for the foreseeable future i am discouraging use of the modern interface and locking users in to the classic experience whenever possible in order to deliver a consistent experience. I have lost faith in Microsoft's ability to deliver a working solution. My biggest question is why release a handicapped product before it is ready? And when is Microsoft going to get it together and finish the work on the new UI? Should we all just put off any new Sharepoint site until MS commits and does their job or move to a better solution hosted by another provider entirely. After 10 years I am quite frankly ready to say goodbye to Sharepoint.