A fair question. It's system stability that I prize above all else.
I believe (but do not have proof) that when macOS boots up it calls the AMD Graphics package (kexts) if you have AMD GPU(s) installed OR it calls the NVDA Graphics package (kexts) if you have NVIDIA GPU(s) installed.
As long as the NVIDIA GPUs are pre-Maxwell (Kepler and earlier) and the AMD GPUs have featured in other Mac hardware releases, then I can expect Apple-engineered stability.
Mixing Nvidia and AMD GPUs is bad as we all know. This could be because macOS is calling 2 different Graphics packages (kexts) concurrently which causes kernel panics, IO conflicts and generally unreliable/unstable graphics performance.
Opencore, flashing graphics cards, installing third-party kexts all affect stability in ways that I can't easily diagnose.
I've got a pretty white Mac Edition HD 7950 in another cMP which I only bought because I forgot that the last time I owned one, it intermittently caused screen glitches 3 times an hour. This GPU is "Apple-blessed" (aftermarket) and it is truly a sub-standard GPU. The problem here is that if you flash this 'official' ROM to a cheaper (PC) release of the HD 7950 - you've still got rubbish.
So in summary - endless trial and error have led me to the most stable GPU solutions for native support with no sacrifices and the formula is simple: Pair an old and gutless Apple OEM GPU with a more modern powerhouse GPU of the same team.
MY FAVOURITE COMBINATIONS..
NVIDIA: GT-120 [SLOT4] paired with a GTX Titan [SLOT1] (or Quadro M4000 [SLOT3] if you are determined to keep both of the x16 slots free)
AMD: HD5770 [SLOT3] paired with a RX 580 / VEGA 56 [SLOT1] (WITHOUT a PCIe expansion chassis - CUBIX or NETSTOR - you WILL lose a 16 lane slot to an AMD GPU).
It's worth a thought