I think you can DO the frog kick in any fin. It's extremely difficult to LEARN in a highly flexible fin. When I took Fundies, I started in a pair of Razors, which are cheap, very flexible fins. (I was using them because non-split fins were required, and they were the only ones we had.) I couldn't get the movement of the kick right, no matter what I did. The following day, I dove in Jets, and got it immediately -- the proprioceptive feedback was so much better.
I have since done one dive in split fins, and I was able to frog kick, although it felt weird and wasn't as efficient as my paddle fins are. The advantage of a stiff fin is that any motion of your foot or leg is transmitted immediately to the water; the disadvantage is that it can be more exertion. And I have found Jets to be very poor fins for full-leg flutter kicking -- it's just too much work!