Safety is a top priority for me. I liked the Corvair partly because it has a great safety record. Most of the few failures were in engines that were built differently than WW suggests.
Can't ignore the safety record of the Rotax. Plus, there are big bore kits if I need more power(probably not).
I will be taking the advice given here. Before this thread, I couldn't find any good reasons not to put a Corvair into an RV-12. Anything I found basically said "Vans wouldn't approve of that", or "it's perfect, why change it". But, the later responses were great and valid reasons.