Couple of gripes I have so far with Challenge:
Defending: Almost seems impossible to contain certain teams. The amount of broken tackles seems as bizarre as the frequency of offloads (which are often thrown half the width of the field with pin-point accuracy). Will try to play with the sliders a bit to tweak this tonight.
Passing/Catching: Watching your scrum-half throw the ball past everyone by 15m is heart breaking when you've spent 5+ phases building up some territory. It's also particularly frustrating when players watch the ball fly past them without even attempting to catch it. Obviously this happens less when you're timing passes and not sprinting, but there are few moments where the physics are just off.
Difficulty: Haven't seen this issue brought up so much, but having gone through a Career mode already, and a couple of competitions, I've noticed a ridiculous step up in difficulty when facing teams like Crusaders/Hurricanes and the AB's. Most rucks are turned over ridiculously easily and they're impossible to stop from scoring tries when they're in possession. Seems odd when their rankings are not far off the other teams (in Rugby 15, and Quad Nations). On a couple of occasions, when awarded with penalties, they'll tap and play out of their own goal line, flaunting the fact that a try will be scored after 20 off loads. Playing on Normal difficulty so the difference is quite noticeable when it happens.
Kicking: Anyone found out a decent way to clear the ball? I've been punting with 9 recently, because as soon as it finds it's way to 10 or 15, the defending team has already smothered him.
These are areas I find will limit my long-term interest in the game. I must say though, that it's an absolute blast playing with a friend at home. I'm looking forward to trying some online gameplay too.