I think the plasmoid works mainly well. Most bugs were fixed, but I still notice that the email list sometimes does not show all unread emails. The number of unread emails is shown correctly "6 unread messages", but the list shows only 3 emails for me right now. Clicking on "check mail" does not change anything, but if I kill plasma-desktop and restart, the Ultimate GMail Feed shows the unread email list correctly. So somehow the update logic is buggy somewhere. If I will be able to provide reproducible steps, I will send another email.
@kecsap