I agree. It takes the user out of their intended action (using your product) and puts them somewhere distracting (their email). I've also seen it confuse users

I agree. It takes the user out of their intended action (using your product) and puts them somewhere distracting (their email). I've also seen it confuse users who aren't used to it. It's great from a tech/security perspective but I wouldn't put it into my own product for those reasons. I definitely would not make it the only login mechanism.