One of my fighters just competed recently against a guy who used the Axe Kick repeatedly in the fight. In 5 rounds, I think he threw about 2 each round. Only one landed, at it did not appear to have done much damage to my fighter. On the contrary, the guy was caught with one leg propped on my fighters shoulder for a few crucial seconds. Luckily for that guy, his height advantage made the reach such that my fighter was unable to really put together an effective counter before the guy was able to get his leg of my guys shoulder.
I personally think that it *could* be used effectively, but it's far from a go-to type of kick.