The powershift is merely a multiple set of planetary gears with multi-plate clutches mounted around the outer circumference of the gear sets. Oil under about 300 psi pressure is directed into the appropriate clutch packs by the spool valve (which is operated by the transmission gear selection lever), which clamps the plates together and thereby selects that particular speed or directional planetary gear set.
The Cat powershifts are pretty robust, and planetary gearsets spread the load, unlike simple spur gears in single-countershaft, direct drive transmissions.
The primary weakness is likely to be in the transmission pinion, and the bevel/ring gear on the cross-shaft. The bevel/ring gear is unsupported adjacent to the pinion, unlike some bevel/ring gears in truck differentials - so the potential is there for the pinion and bevel/ring gear to spread apart under extreme load, and shift the tooth contact pattern, to the point where teeth may fracture.
The steering clutches are more likely to give trouble with a sizeable power/torque increase, and the final drive is only a single reduction - a straight-cut pinion on a straight-cut bull gear - which is weaker than the double reduction final drives of the bigger Cats.