Lets look at it this way. Back in the 80's, OMC labeled all their smaller 2 strokes as 100:1. At that point (and on to current), the technology is sophisticated enough that the motor will run forever on 100:1 without a problem. But, engines started blowing up left and right, and OMC soon regained their senses, now labeling all their 2 strokes with 50:1, and backdating that label to cover all the 100:1 motors.
Now, if the motor could run just fine on 100:1, why did many pop? The answer has nothing to do with the motor running lean, but in fact, not even being run at all. The 100:1 did not leave enough oil residue on important parts of the motor to be stored very long, so motors that didn't get used on a weekly basis started developing minor rust and corrosion issues on vital parts, and would then spin bearings or pop upon being run again.
Fully syn oil isn't going to make a lick of difference in this case, as no matter how good it is, if there isn't enough of it to coat the innards of the engine, it won't protect them.
Does Yamaha have this problem? I don't know, but I do know a few people that really know outboards, who run them on 50:1, especially if they aren't used on a daily or weekly basis. Were the motor mine, I would probably run it 50:1.