I've had no problems whatsoever with the ceramic ball. I haven't heard of any issues with anyone else's either.
The tip will always be too delicate for whatever task actually breaks it. What do you plan on doing with the tips of your knives? Contrary to what many say, the Umnumzaan's tip is actually thicker than the Sebenza's--I'm not really sure how they can think it is possible that it would not be.
The perforated washers don't really "feel" much smoother. They hold lubricant better though.
I don't have a Startac, so I can't comment on that.
My Umnumzaan has maintained a consistent level of smoothness. I have been unable to note any degradation of smoothness. I'm sure if you crank down really hard on the pivot, though, it could become harder to open than it was before.
The Umnumzaan is one of my favorite knives, and it still manages to regularly struggle with my Sebenza for a place in my pocket.
The Sebenza, all things being the same, will always be a better slicer.