That's actually a bad sign. If you can't contact them, what do you do in the event the person lending the car is a scammer?
They did always reply to my e-mails eventually, so it's not as if I can't contact them. If the car-owner ends up being a scammer and I do not hear back from the company, I would deal directly with my credit card. In any case, it's not as if the company can send me a replacement car right away, since they do not have a fleet of their own.
My bigger concern would be if I had an accident using a rented vehicle, but even then, I would be much more concerned as an owner than renter.
And like with any peer to peer service, I would recommend only renting from owners with significant positive feedback.