They could for instance have a generalized terms of service that does NOT cover works licensing, and works licensing be its own agreement which is not subject to change except with more proper controls than an auto-agree if you use the platform any time after they unilaterally update.
Who owns the audience and content at the end of the day? Legally, its still the content creators copyright, but with an unlimited irrevocable license Substack de-facto is co-owner. The audience is entirely built on their platform - the email list looks like it'll be owned entirely by Substack since its through their platform - no mention of this in the ToS. I doubt there'll be an export feature. They may legally protect their rights to "your" subscribers.
If people are already registered on Substack and have payment method in place then onboarding is smooth.
What are the other paid-email failures from the pile?
It feels like you're just charging for a paywall and a mailchimp list... maybe thats enough, but given the amount of failures in the space, it feels like just another to go on top of the pile. Maybe its the right mix, and they have some big plans?
One issue from the Terms - if you post something publicly, they can keep it published and licensed to them forever, even after you delete your account. I'd think for such a high price, and a high emphasis on content creator control, you'd not take away the legal rights to delete public posts.
Also pretty disappointed at the arbitration agreement and unilateral terms of use updates (they can update them at any time, and if you don't agree you will be kicked off the platform).
[edit: its certainly easy to see the value of something like this existing - just trying to understand why this is different than the others]