Hey thats a great question.
I think first of all its good for there to be a few open source projects and options out there, a vibrant ecosystem provides more choices!
Mautic is a solid project thats been around for a while, and has many many features. Our focus is much more narrowly on customer journeys, afaik mautic has added that more recently but it wasn't the core focus.
Beyond that, we want to be easy to self host; we're focusing more on integrations with customer / product data - we've started off with posthog but plan to integrate with segment, Rudderstack, mixpanel etc, as well as databases and warehouses. And "journey testing" is important to us.
At previous companies we've worked at we've consistently seen that marketers/ pms / engineers sort of test emails, sms in production. Issues are caught after the fact, and the big commercial players don't provide too much tooling here. It would be really cool if we could simulate what would happen if specific events are sent, on specific dates, and see what messages those users would receive, we are building a set of features to help this too.