Sintering is good when you only need surface hardness, not core strength. Its a cheap way of making complex 2D shapes that do not require a lot of shear strength with cheap tooling.
Examples in my line of work include valve seats and tooth belt pulleys - If you have to fully machine the part, then it is way more expensive to make than pouring powder in a die, pressing it, then cooking it in a Furness. I'm guessing this is why Mato (Taigen, etc) have moved from machined brass to PM 'steel'.
As Jerry points out, the sloppy fit of the Mato (and to a lesser extent the old Asiatam) gears is the number one contributor to this designs problems. Any slop reduces the unit area available to spread the load from the gear to the shaft that was already available, which accelerates the wear.
Honestly Dan - using emery cloth to polish those shafts is just reducing their diameter by a few Microns, and while it may make the shaft smoother in the short term, its still opening up the clearance between the shaft and gear - its not going to help the long term durability. In an old car you can run thicker oil once the bearings start to wear, in our toys this really isn't an option o.O