G'day LRT,
Everyone knew I wouldn't be able to help chiming in on this one.......and here I am

The greenfield drive is not complicated, its elegant! It looks complicated because it takes up more room and has a belt flailing around.
The earlier stuff with the welded frame (pre- the mecano models) are the most elegant of the Greenies (from an engineering perspective).
There is a history of fatigued and broken axles and fatigue-ing the square section clutch drive shafts in the Tractor models but personally I haven't had the problem. Sure, I've wrecked a few machines that have but haven't broken one personally. My theory is that the shafts are fine if not insulted (bent) by misuse and ham-fisted operation. If they are, fatigue failure is inevitable.
And it's true that the later machines have stronger axles and the shorter square shafts are much less prone to failure but I'm still flying the flag for the earlier machines.
Cheers,