Alright, I played around with the diagonals on the windows game controller utility and found the following:
There is some real inconsistency in a partial press. If you press the d-pad all the way down, it will register a diagonal, after registering one of the adjacent axes. The key being, you have to press it ALL THE WAY down for it to consistently register, and 90% of the time, when it actually registers a proper diagonal, it registers an axis press first, and the only way I could get completely accurate readouts was to maintain constant pressure on the d-pad the entire time, which is not how I play, but it might be how you play (that is, pressing the d-pad down with consistent pressure and only sliding your thumb over the surface).
I tried 20 "taps" on each diagonal, and these were the number of times for each direction that it registered properly, according to windows (that is, display the proper diagonal on the POV hat without displaying an axis press)
Lower left: 3 out of 20 times
Upper Left: 10 out of 20 times
Upper Right: 2 out of 20 times
Lower Right: 5 out of 20 times
I could see someone getting decent results if they're used to never pulling their thumb off of the d-pad ever, as maintaining constant pressure without ever letting up resulted in consistency. In addition, if people press their d-pads much, much harder than I do, they could presumably press all the way down each time. However, for those of us who are used to tapping and releasing, the numbers show that this will not register proper results, unless mine are defective.