What would you list as the main source of failures in your waters?
I'm probaably not the best person to ask. Most of what I break on the new Drives is something I've altered. Most problems I see long before they become failures. Like spark plugs, better to change them before they get too worn out to do their job. For instance, pedal adjustment holes on the drums elongate long before failure (unless the pin doesn't get fully inserted). About that time you can reverse (and regrease) the drums instead of ignoring them until one breaks at sea.
About the most frequent problem on the older (pre-'09) Drives is probably the same one you have -- Allen screws backing out and mast holes wallowing -- ultimately masts dropping out. Some of those screws don't seat fully, causing a problem to develop early for some. Similarly, I used to go through drums pretty fast with the pre-'08 Drives. Both of these issues have been essentially resolved with the redesigned (V-2) drums and sprockets.
IMO, most failures stem from users 1) not paying attention, 2) not knowing what to look for and 3) not knowing how to recover from it. The Drive then becomes less reliable for them. If something actually breaks on a Drive, most folks assume the Drive has failed and stop using it -- yet it's almost impossible to render the Drive inoperable. I managed to do it only once by simultaneously throwing both fins on one of my early linkage screw-ups -- couldn't get any propulsion out of bare masts. Broken pedal, crank arm, drum, chain, cable, mast, fin -- the Drive will still run between 50 and 90% thrust. I break one of my altered masts every once in awhile. It's no big deal to strip the dangling fin away and come in on one fin -- the properly adjusted remaining fin still delivers about 75 to 80% cruise speed on 50% thrust in spite of the boat's rock n roll motion.
As Bob implies, if one takes good care of their Drive (inspection, lube and adjustment) and knows it's capabilities, it will take pretty darn good care of them in any water.