The issue I have with Substack in particular is that it was essentially founded around political newsletters. I'm not sure, but I think Bill Bishop might've been their very first newsletter... and he writes about Chinese politics. Of all topics, that is one where people might have good reason to want to be able to preserve their anonymity. Of course, you could get around this by signing up for different newsletters with different email addresses, but that makes it unnecessarily difficult to read paid subscriber-only posts. I feel like the company took a wrong turn in adding social network features instead of keeping the focus on individual writers and their individual audiences.
Also consider somebody who wants to engage honestly and openly with mental health related content without risking that being dragged in to conversations elsewhere via somebody reading a unified profile.