The way you use your TUs almost exclusively on shooting. In normal (non-RF) gameplay, TUs are spent on moving, turning, ducking, getting back up... And shooting. Clearly, that last part is what you want most, right? Shooting kills things. Turning doesn't. With the old RF, you can theoretically use all of your TUs on shooting if and whenever you have a shot at an enemy during their turn (it doesn't shoot if it can't hit). Didn't get any RF this turn? Don't worry, your TUs are still there, for you to spend on other things as you see fit, and you'll get a full tank of TUs on your next RF opportunity anyway.
So the unfairness comes from the way RF optimizes your TU usage. Again, the fixed penalty is there to compensate, but it really doesn't hold a candle to the saner RF system in 2.3.