I haven’t come across better mail UI than Inbox. Inbox got demoted, new Gmail got promoted. If what I was told is true, then at Google, data beats opinions, that is, they have data to back up those decisions. Now, why am I not convinced this is the case over here?
As one example/anecdotal aside: replacing a link to all mail from/to a contact on mouseover of their name with....links to Google Keep, Hangouts, etc. There's no way the use case data led to a change like this. I mean, how could it?