Experience from two current games considering "Uber-Diplo" with the Axis: in both games I used it, but not most intense, just what was over after securing Spain. While influencing the SU didn't went very well and I had to waste many MPPs before finally declaring war in Aug. 41, this outcome could of course change with a little more luck. Nevertheless it's a dead race, unless you're planning to do a Seelöwe and delay the joining until 42.
In my other game I was far more successfull against the US, and I managed to keep them at 50% until early summer 52 so far, especially because my opponent didn't counter (which is of course the worst possible reaction). US weren't able to send any convoys to GB, and their research has been totally crippeld, while the UK lacked the necessary MPPs to deal with the Axis alone (and hardly being able to counter).
That's why I resume Uber-Diplo to be OP. After receiving many complains about it during my games (and after KZ is missing anyway, hehe), I recommend to abstain from it for both sides, or at least to implement a houserule, where both players agree if they want to use it.