Postmatic Basic does send new posts to your subscribers (which they can reply to to leave a comment). It also handles comment notifications (much like Subscribe to Comments except with replies enabled as well).
Overall you can think of it as an engagement platform: it delivers your content, and makes it easy for people to have conversations around it.
The paid version of Postmatic is more of a Mailpoet replacement. You can read about Postmatic Basic and the differences between it and our paid plugin at our website.
In a nutshell though: use Epoch to replace Disqus. Use Postmatic Basic to replace Subscribe to Comments and/or Jetpack.
Best,
Jason