Bigger gears, better lubrication, and the biggest one of all, tapered roller bearings are what set the M5OD apart from the others.
The most common failure points with the Mitsubishi transmissions are the input shaft BALL bearing (which takes the rest of the trans with it), and wasted 5th gears from poor lubrication. Almost any RBV with a Mitsubishi that's parked because of tranny problems I would bet $$$ it's for one of those two reasons.
The M5OD on the other hand... All I really ever see killing them is an owner neglecting to fix a leak (or at least making sure it stays full of fluid). The three small rubber shift-rail plugs on the rear of the shifter housing are known to leak after they've aged some. There's some info in the Tech Library on replacing them with metal plugs.
Although if you have 2.3L or a 3.0L with the M5OD and you can get your hands on a 4.0L M5OD then you can swap out the internal gearing and get a lower 1st second and 3rd gear.
It's the other way 'round... If you have a 4.0L, you can get lower (wider ratio) gears by swapping internals from a 2.3 or 3.0L one into your 4.0L case.