From a quick read of their agreement, it seems like the payments are only made when you are actually making a net profit, whereas a traditional loan would require mandatory payments or interest would accrue.

This seems like a middle-ground that is better suited to starting up companies that most likely won't be unicorns. They only get paid once you get to a "comfortable" spot, financially.

Another consideration may be that in the event of business failure, you'd still owe money on the loan but the Shared Earnings agreement would essentially go away since the business is no longer viable. Not sure about this, but this is what I'd imagine would be the case.