Substack, AFAIK, doesn't let you own your own domain. This is arguably less of a concern with an email newsletter than a blog, but I confess it'd be enough to make me point back to Ghost, which added full email newsletter management -- complete with Stripe integration, if one wishes to charge -- in v3.

(Also, I confess a bit of hesitance with any platform whose TOS restricts content for any reason other than legality, which IIRC Substack's TOS does. If I started a fiction newsletter and wanted, with appropriate communication to my audience, to send out a piece that contained graphic sex, I don't really want my hosting provider to tell me that's not allowable. It's surprising how hard it is to find places that close off that possibility: as it turns out, the vast majority of the commercial internet is not, in fact, for porn.)

+1 for Substack, I think it really helps writers focus on getting their words out instead of worrying about distribution.