IMHO, services like Hey is for people who don't know how, or don't want to bother, to set proper inbox rules (it has many other features for sure but organizing emails is one of the biggest selling point as far as I see). But for me, I just want a reliable, dumb, boring email service without any "smart" features. I will decide what to do with every type of incoming emails myself. Fastmail gives me exactly that.
The magic of Hey is the user interface. The “opinionated” nature of their design choices.
To me, their workflows mostly made sense. For me it really really smooths out my email workflow into a logical set of steps.
Other people may not like that flow, and Hey wouldn’t be for them.
So I’d counter and say Hey is for 2 groups. 1. Those who don’t know how to set up complex rules 2. Those who don’t want to, and Hey’s flow works with their own.