I don't agree. The startup is guaranteeing that the customer won't pay more than one month's rent, which they very much can control. The company is on the hook

I don't agree. The startup is guaranteeing that the customer won't pay more than one month's rent, which they very much can control. The company is on the hook in cases where they can't find a replacement, but that's not the customer's problem. It's very possible that the service is priced too low in order to get traction, but there's nothing inherently unsustainable about taking on risk (with a reasonable cap no less); it just comes down to what the company's placement rate ends up being.