Wow 4Ax5 (JF) to iMCF in so few frame seems way harder then SC4, maybe that's what I did before when I was doing it in a match. This reminds me of CH d/f+2 PEWGF in Tekken where you have exactly 13f to pull off a 13f move, hard stuff. Can you still RCC into 3B or 1B+K, or can you only do stuff like 66B?
I thought I tried MCF, bA on Pyrrha but I'll look at it some more, but I do know it definatly works on Pyrrha Omega, size 2 3 4 5 CaS, Xiba, Ezio, Dampierre, Ivy, Siegfried, Nightmare, both Palakolos, Lizardman, and Tira. I don't think their is a special case but as I said, I will look into it.
Another question would be what does the exact orientation of he character have to be for the move to hit, when I use SDF~KK I sometimes am able to do bA to the opponents side, this may help. But CH iMCF can sometimes make your opponent fall towards you left side, don't know if it's the terrible physics or if it's supposed to do that though.