I thought this too a few months ago when I was messing around with it. I was trying CH, and on crouching opponent, but it would never combo if the CPU was set to block the 2nd attack. I know that if you don't have the CPU block the attack that the combo meter says that it's 2 hits, but those are not guaranteed since I couldn't get it duplicated with a blocking foe. Anywho, it's examples like this that show you that you really have to test something thoroughly before telling people when something is 100% guaranteed.
Ah yeah fair enough, I was not aware of that, my bad. I did a second test after reading your post and this time I recorded the string and tried blocking it myself. I did manage to get it to combo this time actually, but only when the opponent (me) had it's back to a wall or edge, which remedied the knockback problem from a max distance FC 3[A+K]. If FC 3[A+K] hits while it's falling, the hit stun window will be big enough for MCFT B+K to connect if the opponent is close enough.
How easy it is to pull off seems to be character specific also. Was almost impossible on asta and rock, a bit hard on mitsu, and relatively easy on amy, taki and ivy.
So in closing, it works afaik, but is not a very safe/viable option imo.