Apple identifies the card by the product ID, if the driver found a different product ID, the generic framebuffer is used.
Not all RX-560 are equal. Some require more power than others (nearly twice as much) and an aux power cable, which is not a big deal. But it is convenient to be able to just plug in the card and go without fiddling around with a power cable, especially if you plan on removing and reinstalling the card very often. Or if you ever plan to install a 2nd card, even temporarily, which needs aux power from both of the available aux connectors. And it's always nice to have a card that requires less power, no cable, runs virtually silently, etc, especially if you ever decided to use 2 RX-560 cards (for more than 3 monitors). The MSI RX-560 ITX 4 GB card (the one officially supported by Apple) is such a card, and it's cheap (I paid $130 for a new one.)
There are, or were (not sure which of them are still available new), numerous versions of the Sapphire Pulse RX-560. They fall into both categories I discussed above, so people considering them should check out which version they are getting - if it matters to them. Again, to most people it probably won't matter if it requires an aux power cable or not, whether it's a 45 watt, 60 watt, 75 watt, or 90 watt card since they are all very low for Mac video cards, and the difference is less than 15% whether it is a 16 cu or 14 cu card. There are also slightly different clock rates for different cards, but those differences are slight. But if you do care, then look carefully at the specs for the Sapphire card you are considering.
I'm not trying to be argumentative, and I'll bet you are right that others are compatible. But Apple does not guarantee or state they are. They state they "might also be compatible".
Is it a DUAL PCI slot card or thicker (2.5 slot ? ).
