Interestingly, I have never tried 6 or 7 non-GPU cards before, I always had one or more GPUs in my tests...
So first I disabled the option ROM on all PCI-E slots, and tested 6 and 7 RAID cards, and I have POST with 6 and 7 cards!
If I do 4 RAID cards + 2 FC cards I have POST, but not with 4...