Shopify Mail is sorely missing the ability to target based on customer interactions with marketing campaigns. With MailChimp I would often target people that did not open a specific email. I would also target those who consistently open emails (like only people that have opened each of the previous 3 emails). But there are endless possibilities of targeted emails based on marketing interactions. I suggest Shopify take a look at MailChimp.
PS - Yes, I’m aware there are Apps for that, but I have too many Apps as it is.