As it has a proven track record it is unlikely for any issue to arise on the Bustabit server therefore third-party associates would most probably be where to look. If any outage were to happen at ActuallyFair (correct me if I am wrong) it would bring a cloud of uncertainty to all rounds that have finished on Bustabit until the connection was re-established.
The downsides are that the system is more complex and therefore more difficult to explain, especially to less-technical players, and that an outage at ActuallyFair could interrupt betting.