Before bustabit accepts any bets to a round, it connects to ActuallyFair and collects the piece of information it needs to calculate a game result (the bust point). Only then does the round open for bets. So should ActuallyFair not be available, bustabit would be paused until it's able to connect. If it was done in a different way (e.g. Bustabit could start without ActuallyFair) it would open the possibility of cheating (i.e. bustabit pretends it couldn't connect to ActuallyFair because it didn't like the roll outcome).
All that said, ActuallyFair has been specifically designed to be highly available. Customers are provisioned with their own dedicated instance, which is firewalled so only the customer can connect to it (i.e. making it immune to DDoS attacks and the like) , and it has automatic fallback in a different physical location. So I hope to not have to eat my words, but availability should be very, very good.