There are only two Rules actions that haven't been fully ported to D8 yet, and "Send mail to all users of a role" is one of them. When it is finally ported, it should provide the same capability as in D7.
There's a patch in the Rules issue queue which ports this action to D8 - you can try out that patch to see if it solves the issue for you. See:
Port Send ...
I've recently built a subscriber module with a small piece of custom code. I prefer working with webform handlers instead of the Mailchimp module.
In a custom module add a folder src/Plugin/WebformHandler and create a file newsletterSubscription.php. Add the following code: