Further extensive testing- using an older 7-slot Magma Chassis as well as my newer 13-slot chassis, suggests that there's a problem as soon as you try to add more than 2 UAD-1 cards to the system, even when spread across 2 chassis and the remaining free PCI slot in the G5.
The best I've managed is 2 UAD-1s and a Powercore element.
Anyone else found this? Highly frustrating to be given the huge flexibility of the Zed R16 for mixing on one hand, while losing 2 of my 4 UAD-1s...