‘Challenging’ (when applied to a non-interactive work or aspect of a work) is one of those compliments that seems to me to say more about the critic than the subject: what does it mean that a piece of fiction is ‘challenging’? Whom does it challenge? Why is this challenge a good thing? Is the challenge done well and fairly? And if so, wouldn’t it be better just to say that the work was ‘balanced’ or ‘thoughtful’ rather than ‘challenging’?
Also, when was the last time you heard someone describe a work that took an opposing position to his own as ‘challenging’? Would, say, a staunchly pro-gun-control critic call something like Death Wish ‘challenging’?
Probably nine-times out of ten, to call something ‘challenging’ means only, “this seems to me to poke a finger in the eye of someone I don’t like.”