It's not guaranteed at all, it just means they have the request for a ride in their queue and when it gets closer to the requested time, it will go out to drivers to pick up. And yes, a driver can cancel at any time prior to you getting in the car. I had a pick up request at my airport and it was a busy time. I had 5 rides cancelled before I got one who actually showed up. Then it cost me $85 for the same ride that cost me $21 when I was dropped off. I could have hailed a taxi for $50, but it was at a different place in the airport and I was too tired to walk there with all my luggage. Surge pricing.