As I see it, One of the big advantages that ki users get is that their powers aer exactly what they want them to be. They tend to do things like make you hit harder or more often. They are the very sort of powers that player who are allowed to make themselves would want to make.
I don't really see a problem with letting players make their own powers as long as I look over them to make sure they did it correctly and they aren't too crazy.
Since Ki techniques are really expensive, and you use your 60% of your points dedcated to combat to buy up your ki traits, it doesn't seem over powering to me.
However, I was wondering if any other GM's out there ran into dificulties letting PC's have a free reign with their powers.