So I've been playing the campaign in Rome 2, and in every battle, even on hard difficulty, Roman legionaries just seem to steamroll everything in their path. But then again I'm also aware of several historical battles where Hannibal was able to completely route Roman forces, especially where he defeated 90,000 Romans with only 25,000 men to himself (can't remember the name of the battle, sorry). That being said, were Roman Legionaries actually far superior to their Carthaginian counterparts, or is Rome 2 just making the game easy for new players?
I'm on my phone so will add more sources when I get home, but this is an area of expertise for me (or at least as close as I come).
When you look at the military record of the Carthaginians, it's certainly not very impressive. Apart from the Barcids, there isn't exactly a glorious roll of victories in their history- they struggled against the Sicilian Greeks, their own mercenaries and the Romans. Just considering the Second Punic War, if an army wasn't led by Hannibal it usually lost to the Romans. This wasn't, though, down to troop quality. If you look at the various individual parts of the Carthaginian army, you see well-designed cogs. The Numidian cavalry is repeatedly praised by the ancient sources; the Spanish and Celtic mercenaries were ferocious warriors; the Libyans could give it out just as well as the Romans. The problems arise when you consider their position as part of a greater whole. Hannibal's army operated together in Spain for a long time, first under Hamilcar, then Hasdrubal and finally Hannibal himself. They were used to fighting and functioning together, and this unity and training helped give them the edge against the far more homogenous Roman forces they faced. But most other Carthaginian armies were not so experienced or long-serving and lacked the time to become so. Look to Iberia, where what should have been several powerful armies were regularly given a pasting by first the elder Scipios and then the soon-to-be Africanus. Though varying contingents gave a good account of themselves at various battles, the whole- a whole made up of different "national" components which spoke wildly different languages- was considerably less than the sum of its parts. Note that the events which led to the defeat and death of the elder Scipios sprang from strategic miscalculation and Iberian treachery, rather than a Carthaginian victory in a pitched battle.
There is also the question of leadership. Goldsworthy posits that, on balance, Roman commanders tended to be average in ability but far exceeded their Carthaginian equivalents in aggression, something that probably stemmed from the Roman political system and the desire to win glory and career advancement back in Rome. Carthaginian generals tended far more towards hesitation. This is understandable, given the Punic predilection towards crucifying generals that failed, but it resulted in chances being missed that could have turned the course of the war. When the elder Scipios died in Iberia, the Carthaginians had an opportunity to throw the Romans off the peninsula, which would have made Africanus' job far more difficult and could have altered the course of the war. A combination of Roman aggressive leadership (on the part of one Lucius Marcius, an equestrian) and their own timidity ensured they instead returned to pacifying Iberian tribes, a far lower-hanging fruit.
Considering leadership, an important factor to consider is that, while the bickering of Rome's consuls led the Romans to some devastating tactical defeats, Carthaginian generals' inability to get on with other quite possibly doomed their efforts in the outlying theatres. Prior to Africanus' arrival in Iberia, there were three strong armies active on the peninsula, but their commanders actively disliked each other and they operated largely independently of each other. In Sicily, where Hannibal sent a talented Numidian Liby-Phoenician~~ (Muttines, who I have also sen referred to as Mottones/Mattan) to help control the Carthaginian forces, the existing commander (Hanno, off the top of my head) got pretty rubbed up the wrong way when Muttines started winning battles, improving Carthaginian fortunes and generally hogging all the glory for himself. He made no secret of his dislike, dismissed Muttines and gave the command of the Numidian cavalry to his own son. Long story short, Muttines ended up defecting to the Romans who, a little more familiar with the notion of gratitude, made him a Roman citizen. Carthage's position on Sicily was, predictably, soon untenable- the city of Agrigentum fell thanks to Muttines' men, which need not have happened had it not been for Hanno's jealousy.
So, all told, it took some specific circumstances- experience operating together and good leadership- to make a Carthaginian army into something that could take on a Roman army face-to-face and win. Carthaginian armies were not useless and should not be dismissed on the grounds of their use of mercenaries, as some do, but it was far harder for them to forge these armies into world-beaters than it was for the Romans.
Hannibal had around 50,000 men at Cannae from memory. I think RTW2 (caveat - haven't played it yet, but played the fuck outa RTW) is designing for effect, the Romans were very successful militarily so their units are made to be OP. They're designing for a desired result.
Rome was successful for many reasons, on a tactical level they combined an excellent aggressive fighting style with really good command and control mechanisms that allowed them to outmaneuver their opponents.
Operationally they were masters of logistics and could support armies where others couldn't go. At the height of the 2nd Punic War, they were supporting an army in Spain, Sicily, and in Italy itself. That's a massive accomplishment in a world primarily relying on slow ox-carts (the Roman collars were designed for oxen and would strangle the horse if it tried to pull a heavy load) or pack animals.
Strategically Rome was fantastic at integrating conquered nations. Consider that half of every army in the 2nd Punic War was supplied by allied states. And that even with Hannibal wiping out 3 successive Roman armies (the last in a massive bloodbath), the bulk of those allied states stayed loyal. The allied states even revolted in the 1st C BCE to gain Roman citizenship not to break free. This meant their available manpower was much larger and they could drown their opponents in new armies. I think Hannibal probably had a moment when a 4th Roman army showed up instead of a peace treaty after Cannae.
Part of Rome's success and failures can also be laid at the feet of the elites. Their hunger for military success (which brought electoral success) made them very aggressive which worked for them - except where it didn't. But because of the Roman inability to understand words like "surrender" or "fair negotiation" meant they'd just send another army under a different leader to kill the fuck out of whomever didn't have the decency to be conquered (and exploited).
While their is a modern trend to view these elites as dilettantes and attribute all the success to professional centurions (who are often likened to sergeants) I think this sells them short (and also projects our modern belief structures). While there was no formal training to be a general, each almost certainly had military experience. They'd have served as "staff" for family members or friends and observed and learned. Even the "professional" centurions may have been in effect political appointees. Goldsworthy pointed out that we have precious few examples of men promoted up through the ranks. Centurions were used as low-level ambassadors or handled independent missions. He puts out the theory that most were from the equestrian rank and were appointed. It would explain why Augustus personally promoted centurions (can you imagine the Chief of Staff of the US Army having to look after the promotion of sergeants?), these were important men who just weren't as rich as the senatorial class.
