The new UI is fantastic, a big thank you to the F-Droid team for their hard work.
One paper cut I ran into: if an app was installed from the Play Store, it is not possible to update it via F-Droid. The old UI clearly communicated this, whereas the 0.103-alpha3 shows the update notification, attempts to install it, and silently fails, returning to the updates page with the app still listed as an update candidate.
If you go to notifications, you'll see that the install failed "due to an unknown error".
Let me know if I should report this in the GitLab issue tracker.