it seems very hard to aggregate this in practice

Yes, it is. However there is, for example, the Cochrane Collaboration which is dedicated to exactly that.

Why should I believe that a company started by futurists and entrepreneurs would be up to this task?

You should not. I am not arguing that MetaMed is better than everyone else or even that it is very good. I am arguing that it's not evil, not dangerous (relative to the usual baseline), and a useful thing to have around.

It's goal is not to provide you with THE TRUTH, it's goal is to give you a digestible summary of the current research on topics of particular interest to you. Often this summary functions as a second opinion, or it could provide the context for making medical decisions. It is as fallible as the rest of contemporary medicine.