How hard is a heart transplant to perform?

Relatively easy. Well i never did one, i'm not a surgeon and of course they train for many years. However, it is technically probably easier than some of the other surgery they do as you are sewing fairly large structures together. Working with smaller structures such as a coronary artery would be technically more challenging. Then again, there is a lot more sewing involved with doing the transplant.
Technically easy. The connections to be made for a heart transplant are pretty straightforward for a cardio thoracic surgeon. It is the judgments that are tough. Which heart? Will it fit? Will it start again? How long can you wait? Which patient ? Which medications? Lots of places for things to go wrong - but they usually go right!