The only powertrain problem worth noting is that on the 4.6L, the ZF 6 speed auto has a higher failing rate than the 3.8L. When I drove one, I thought the V6 3.8L had plenty of power. The 4.6L V8 is much more thirsty, but it gives you over 80 more HP. To fully use the V8's power, you need premium gas or else you will lose 12 HP. The 3.8L has a more reliable transmission.
The main problem affecting the 2009 Genesises are brake pedals that become so spongy that it is difficult to safely stop the car. There are 6 reports of crashed because the brake pedal went to the floor and didn't stop quickly enough. No deaths that I am aware of. It is unlikely to have this problem, but it is the number 1 problem people are having.