1. I'm not too sure, each state varies.
2. There is a high failure rate because most owners will do no credit check and have no qualifications for a tenant. If you have the down payment handy, they'll sell to you no matter what. When I first started I did that, I had my tenants leaving after 4-6 month. I started to some qualifications which has definitely brought me much better tenants, but I also love selling to somebody that has a trade. Those are usually my best tenants and rarely give me any headaches.
3. I can't think of a huge downside that makes me want to stay away from owner financing. My phone isn't ringing constantly because I don't handle maintenance, nine times out of ten the tenant is actually making improvements to the home, and I can charge a little above market rent because they're actually owning the property, and I can buy cheaper properties that need some TLC, do no work to it and sell it as is to somebody who doesn't mind doing the work in exchange to be an owner of a home.
I do lease options on every single one of my properties, I can't imagine switching over to traditional rentals. Happy Investing!