I don't think it really has to do with the carrier but the condition of the coral (water parameters and care recieved) before shipping and the way it is packaged. I have shipped many through all the carriers and USPS the most and had very few losses, none lately since using a newer preparation technic.
But I only ship a few every so often and only as a hobby, not on a large commercial scale like some over-the-net dealers so I can afford to take the time to prep the coral for shipment.