Cat flat out told him 'never trust a Greyjoy'. She was also pretty straightforward about not getting in league with the Freys to begin with and about not marrying Talisa -- in fact she floated the notion of Frey delivering Robb to the Lannisters, which is what prompted her to suggest going to negotiate in Robb's stead. No one, not even his wife, thought it was a good idea to kill Rickard Karstark.
Sure Robb was well versed in military tactics, why wouldn't he be after being groomed from day 1 by the likes of Ser Rodrik and Ned to command the Northern armies? There's more to being a good leader than understanding the Xs and Os of war, though. Great, he never lost a battle -- that stat is utterly meaningless when you sacrifice 10% of your men early on , drive away the Frey forces, drive away the Karstark forces, and hand deliver your remaining troops for slaughter to the man you just fucked over. The vast majority of his army was lost as a direct result of dumb decision making and poor leadership.
Stannis, as rigid as he can be, does actually consider his advisors' advice, even though listening to Davos and not taking Melisandre to Blackwater is a big reason he lost along with the sneak attack by Tywin and the Tyrells.