I think keeping a single one isn't challenging.
I think keeping a group is like playing with fire. If you aren't careful, you can get burned. If you take proper precautions, it can be great fun!

Adding to what Nick mentioned, aside from aggression and feeding, keeping up with water changes is a third challenge. But I can't think of a fourth. Keep proper numbers, in a proper environment, with good food that isn't overfed, while maintaining a religious water change routine and you should be fine.
I started with a sub-adult group of 7 a long time ago and they did well, even bred! So I got another 30 of two more kinds and the whole thing started to blow up! Aggression magnified their stress, leading to bloat, leading to dying fish, and when I was finally able to curb the havoc I had ~10 left. Really sucked because tropheus aren't necessarily cheap.
I took a couple years off, then got back on the horse and now I've got two groups currently doing really well.
