The flipkick doesn't work too great on a few characters when they are standing.
The flipkick won't cross-over without a blockstring (it does when they are crouching or knocked down) against these characters:
Bane
B2, F2(random, mostly cross-overs), F213(first and second hit), B1, B11, 3, 33, *STAFF 1,F1~4*
With Bane sometimes it looks like you cross-overed but you didn't.
Black Adam
There are none, it's very random and most of the time it won't cross-over.
There is no way of telling if it will.
Doomsday
B2, F213(only the first hit), B11, 1(random, mostly cross-overs), 11(same), 3, 33, *STAFF 1,F1~4*
Sinestro
2, F2, F21, F213(only the first hit), 3, 33
Solomon Grundy
Absolutely none, nothing random either.
I only tested the flying and big characters.