I agree with their use under very particular circumstances, like in your example or (at least in desktop Chrome) for bringing attention to a window that isn't visible. I was referring more to using alerts as they're being used in the SweetAlerts examples (success messages and other "flash messages"). In these cases, UI-thread blocking seems bad, no?

I actually think SweetAlerts looks pretty nice for those kind of in-app notifications, which was the point I was trying to make. (Though seeing comments from people who have dug in further it seems like it has some issues)