So Punishing Kick...
Punishing Kick
Prerequisites
Con 13, Wis 13, Improved Unarmed Strike, base attack bonus +8.Benefits
You must declare that you are using this feat before you make your attack roll (thus a failed attack roll ruins the attempt). On a successful hit, the attack deals damage normally and you can choose to push your target 5 feet or attempt to knock them prone. If you decide to push the target, it is moved 5 feet directly away from you. This movement does not provoke attacks of opportunity, and the target must end this move in a safe space it can stand in. If you decide to attempt to knock the target prone, the target receives a Fortitude saving throw with a DC of 10 + 1/2 your character level + your Wisdom modifier to avoid the effect. You may attempt a punishing kick attack once per day for every four levels you have attained (but see Special), and no more than once per round.
My big question is why is this a Fortitude save and not a Reflex save? To me it makes sense that Stunning Fist is a Fort save. The way I see it its impacting a person's internal bodily systems. Possibly hitting a pressure point or simply disrupting the persons mind body connection, like taking a huge hit might do. But with Punishing Kick I feel that it makes sense that a character is hitting them in just the right way to throw off their balance and use their momentum against them.
Also, looking strictly at the mechanics of the game we also have a stunning fist type ability that targets Will saves with the feat Touch of Serenity. Would it not make sense to have Punishing Kick target the Reflex save, providing players and especially Monks with 3 nice options each targeting the 3 different saves. It just surprises me this wasn't how it was written in the first place.
So my next question follows this up. If I houserule that Punishing Kick does target the Reflex save for my players would those of you with more experience DMing than I tell me: are there any unfair interactions that would make this an unbalancing change to the game?
Why is it a fort save? Because that's what the rules says.
I could see multiple ways of explaining it to be a fort save or reflex save. However, reflex saves are usually to avoid (or partially avoid) something. With punishing kick you've already hit, that's been establish with the attack roll you made. The fort save is because you're trying to kick them in knee (or other weak point) to cause them to fall prone. At least that's how I picture it.
Would it be unbalancing to change the type of save? Probably not. Would I ever do it, no. I don't see enough of a reason to do so except that you have a desire to have monks have abilities that target all saves.
Whether or not it makes sense is a matter of personal taste, though I do agree wih reflex. Might try and get an idea from players first though.
Power-wise though, its fine. Reflex is generally the lowest of the three saves, but the effect here isn't too great (combat maneuvers can do this) and you can only use it a lot if you are a monk or brawler, classes with low accuracy on individual attacks (relatively speaking of course) meaning it will probably miss multiple times. Other classes will hit more often, but could only manage this a couple of times/day.