They weren't particularly impressive in hand-to-hand combat at Cannae. Though those were rather green. Their tactic wasn't as thoughtless as some would like it to be. When you consider the context of their experience at Trebbia. I'm sure they were going for a total penetration of the front line. One would've expected their army to burst through hannibal's thin frontline as they did at Trebbia.
If the Romans were as impressive in close-combat as is suggested, then they should've caused a lot more casualties in the Carthagininan army than they did, despite of Hannibal's superior tactics. They were surrounded, and the outer concentric ring of men would be fighting to the death. In the end, 70`000 romans had died, while Hannibal lost 4000 celts, 1500 iberians and africans, 200 cavalry. It is very likely that irrespective of Hannibal's tactics, his mercenaries were generally better skilled in close combat.