Privacy wise, you're paying for the service so there's a reasonable expectation that they're not mining your emails to build a profile of you. Unlike Google they have no ads to serve you.
> all your personal emails are stored in [someone else's] servers.
Well, yeah. That's true for everything except for hardware you actually own.
They're saying it's better than storing it on Google's servers, not that it's bulletproof.
There really isn't a way to have impenetrable email. It's all about what type and level of risks you're willing to take.
E.g. are you concerned more about rubber stamps or software exploits? Are you more concerned about usage pattern profiles or someone actually reading the content of your messages?
Different people have different priorities and there is no one single best option.
