From what I remember of TNO, it did a better job of scaling the encounter design with BJ's arsenal and potential Perk upgrades as the game went on. The beginning half was mostly normal Nazi Grunts, dogs, and Commanders in large, but manageable, numbers. The much-maligned "bullet sponge" Armored mofos started coming out later, but BJ has all sorts of powerful shit to kill them with by that point in the game.
Here in TNC, I feel like they start introducing tougher enemies well ahead of BJ's power curve. There have been far more of these "David vs Goliath" situations, where I'm trying to fend off multiple of these charging armored guys with (relatively) wimpy dual-wielded SMG/AR peashooters, because it's all I have.
It's been 2 years since I last played TNO, so my memory could be off on how that game was structured, but TNC is certainly the more difficult game of the two for reasons I'm still trying to figure out. I'm not nearly as down on the gameplay of TNC as some others here, but it's definitely different. My favorite parts of TNO were mowing down all the cannon fodder Nazis and TNC has significantly less cannon fodder, but the combat is still mostly enjoyable.