One drawback of AIOs that I've read about is that you're no longer air cooling the VRMs surrounding the socket, which can decrease the life of your motherboard.
They also recently decided to include 4 120mm fans in the package, two push and two pull, which is a pretty crazy amount of air cooling.
If you believe their copy it won’t work on most computers in a tower case, since the CPU isn’t on the bottom and it uses a gravity feed to return the liquid.
This used to be the main problem with this technology and they claim they solved it.
Noctua's cooler is most nice look for me so there's no problem