For me, an additional concern is being tied to this client after using it. Every offline mail client has its own storage formats (mbox, maildir, SQLite, another database or a mix of these), and so moving across clients is not an easy exercise unless the clients support multiple formats for import. I would also like to see how the client behaves when one single mailbox is more than 20GB in size, with at least one of the folders being around 3-4GB by itself. Claims that searches happen in milliseconds need to state the size of the mailbox and the folders (and also include enough variability in the content for the index to be large enough).
I certainly like the idea of new email clients, especially those that integrate with Exchange calender (a weak point for Thunderbird even with extensions). But in my view, building a fast, robust and feature rich email client would take about an effort of 10-30 person years or even more, depending on the feature set. This one seems to stand at a mere two person years, and so my expectations would be quite low (though it's still in beta and states upfront some of the important features it's missing).
P.S.: I'm a supporter of Thunderbird and donate money to the project. https://donate.mozilla.org/en-US/thunderbird/