Appartently there is some electrical incompatibility with the Shark/Sonnet PPC cards and the Mediator, in that the card requires 3.3V, but the Mediator does not provide it. There is a comment by platon42 on amiga.org:
Because it cannot work in an unmodified Mediator *ever* as it's missing the required 3,3 V traces. It cannot work for other reasons, at least not if it wants to have access the the motherboard hardware. mboehmer could elaborate on this, and I'm sure you will find some information about it somewhere.
The following URL has info on the different PCI standards with relation to 3.3V and 5V, and that they should be keyed differently, although some cards are keyed for both slots: