counter hit 214d in the air knocks them down to the ground, they can't air tech. big damage if you can follow it up with 6b. (3.7k+ at least, with an easy combo)
edit: 5996 damage in corner if you use omoihikane (6a 2c jc j2c combo. 5c 2c won't allow omoihikane.) and about 4k damage if you can get 6a j2c dash in 2b
a tip for anyone having trouble with 6c fatal (with 6b 6a 6b) most of the time if you drop it, it is because you used 6b too early. if they can air tech before the 6a hits (which you have to hit them when they're very close to the ground) then that's a sign you used it early. if they tech on ground, then you missed the 6a follow up (waited too long). either way, if you realize you missed the 6b link then just do 5c 2c air combo after the 6a, with a possible dash in, not sure about the dash in though.
so far, for 6b 6a 6b fc combos, i can confirm it works on everyone. the timing is most likely character specific (tsubaki was the hardest, carl was surprisingly easy) and for some characters you can just do walk up 6b and get the link down (mu)