Not sure why the yamaha engines being " Used " is a concern.
My yamaha I installed on my Dominator was used, but it had only like 200 miles of use. So yes it wasn't brand new, never been run... But 200 miles on a snowmobile equals like less than 20 hours, possibly less than 10 hours of engine run time.
Would you tell me that you would pay significantly more for a Rotax 912S with zero hours when you could save thousands of dollars buying one that is known to be running in perfect condition, and only has 10 to 20 hours of run time on it?
Some people I know would actually place a higher value on the used engine because its broke in, and proven.... compared to the engine that is brand new and not yet proven.
Rotax does have dual ignition, but losing ignition on a modern four stroke motorcycle / snowmobile / pwc watercraft type engine is EXTREMELY rare. enough that I would have very little concern or worry about that. My worries would be things like will the crank fail, will the cam chain fail, will a valve break off and drop, will a piston fail.... Or concerns over will the gearbox fail or will something about how the gearbox is attached or some kind of harmonic cause failure of something inside the engine.
Rotax 912 engines are well proven that other than some type of defect, the engine internals should not fail under normal operation. It is because of that, that if I were to fly over the amazon rainforest I would prefer the rotax over the yamaha. But I would feel even better with a Lycoming O-235 than the Rotax....
The Yamaha engines are racking up hours though. They have already long been proven in their original applications. Their motorcycles, their Snow machines, and their PWC's are legendary at being the most durable on the market. But there are way more people flying yamaha now then you might imagine. There are a TON of people putting them in fixed wings now. So fleet hours are going up and issues and problems are being sorted out.
A USED, but extremely low hour yamaha can be put together for less than half the cost of a Rotax, and for some people that savings is critical. But something else to consider is replacement parts and servicing on the yamaha is also a fraction of the cost of Rotax parts and service... and the yamaha engines also make more power at a similar weight.