Okay, well a week later: now that Build 8523 has been released, it actually seems to be working okay! Editing has pretty much reverted to how it was in EyeTV 3, the thumbnails are showing up okay in the editing timeline, and it seems to work like it should. There are now at last more export options, and the picture no longer stutters. Other than two "View" items in the menubar, and the fact that once you start exporting an item you can't stop it without quitting the app, for the first time it actually feels like a finished product. (Note: I'm in the UK, using an HDHomeRun Connect.)