While MailPoet may be open-source (which is great) the problem with MailPoet is that it’s just complicated.
It’s complex to install, and difficult to integrate with other tools.
The documentation is overwhelming, and there isn’t as much education out there to help you. You’ll be heavily reliant on the community forums as there is no full-time support team.
MailPoet is also dependent on user contribution, which means improvements happen when they happen.
Groundhogg on the other hand is a for-profit operation, incentivized to give you what you need, fast! With courses, constant education production, and a super responsive support team.