The counter attack makes combat ridiculously easy. The problem is that otherwise, the combat is horribly broken and random. What is better: Uncontrollable, luck based combat or butchering every opponent without difficulty?
In the game's defense, switching weapons does change the timing on the counter move, adding variety, if not difficulty.
Even with the change of the timing on the counter move as weapons were being switched, guessing the correct timing to perform a counter is just as good as doing it, and that's if you're able to see every attack through to fit with your timing. Which makes surviving the whole game with ONLY the Hidden Blade possible if you have the ability time EVERY single counter correctly, not to forget that you can use that Hidden Blade to do a high-profile assassination during a fight if an enemy is taunting you or is cowering from fear. Not even the game's Big Bad is spared (although he doesn't taunt or cower in fear like how your enemies will usually do)... Let's just say Alta´r and his trademark Hidden Blade near the end of the game is pretty much a Game Breaker himself.
It should also be mentioned that counters from the swords are notOne-Hit Kill; counters from the hidden blade are. Even on the final boss.
It gets better. In the final fight, the easiest way to kill Robert is to just grab and throw him, switch to the Hidden Blade, then stab him while he's floundering on the ground.
And when you kill a couple of guards in quick succession with the hidden blade (grab them both and throw them to the same place), it's likely someone nearby will cower in horror, allowing you to kill them with it. This sets up a murderous domino effect where you can kill 10+ guards with no retaliation.
Assassin's Creed II
The game has an economic system that's completely broken by the villa. Basically, you have a villa in the Italian countryside that people can visit and pay rent to stay, all of which returns a payroll every 20 minutes. You can increase the amount of money you make per payroll by renovating the villa, and fixing up shops can get you discounts when buying things from stores. When all is said and done and you have a fully-upgraded villa, you basically never have to worry about money ever again. It's balanced somewhat by your stake in the income having a cap with any extra money being lost, but the cap is generally large enough to not be much of a worry, and upgrades as you renovate.
The pistol upgrade, which gives you a small hidden blade style firearm. While this weapon takes a while to aim, it will one shot kill any enemy in the game, including bosses. Many bosses past the point where you get this weapon are high up (and extremely difficult to reach at all let alone staying hidden), surrounded by armored guards and other boss level enemies, and/or extremely skilled in combat. The pistol takes all these factors completely out of the equation thanks to its extreme range and perfect accuracy after 3 seconds.
The Hidden Blade's biggest change in II was a little feature missing from version one: the ability to block (okay, they also added a standard attack for it, but that was rather pointless unless you wanted to see some of the spectacularly gruesome combo kills). This may sound odd, but because you couldn't block with it in 1, you pretty much had to be perfect on the timing or lose a lot of health as the guards wailed on you while you were staggered. Before, if you weren't too good at the game, you had to stick with the sword to avoid the previous previous; now, there really wasn't any excuse not to use the Blade. (It does less damage than the primary weapons, but who needs a few more points of damage when you're one-shotting every enemy that attacks you?)
Assassin's Creed: Brotherhood
The three main "breakers" are executions/kill streaks, the Assassin recruits, and Arrow Storms — executions/kill streaks allowing the player to One-Hit Kill most mooks (and in some cases, even mini-bosses) while going on the offense, the Assassin recruits allowing Ezio to be credited for kills while completely avoiding detection (the recruits also potentially becoming one-man/woman-armies in their own right upon reaching the highest rank), and Arrow Storms one-shotting any enemy in Ezio's vicinity.
The player, with patience and a couple hours, can level several recruits up to max experience (and subsequently complete most of the recruit quests as a result), giving the player an unbeatable advantage very early in the game.
The Hidden Blade has a slightly wider window of opportunity than in the previous games, although that is probably a subtle nod to Ezio's experience, while the Hidden Pistol's much faster aiming time (making it an actually viable combat weapon) is a more overt example.
The Crossbow, which is like the Hidden Pistol, except without the stealth-breaking loud noise or severely limited ammo. If you're moderately quick on the draw, you can kill any guard before they raise the alarm, even if you haven't even drawn the weapon when they initially spot you. This makes rooftop archers in particular completely cease to be a threat, since they tend to move on their own.
The Poison Dart Launcher makes poison so much more useful. You can now poison enemies from afar, with almost no chance of a hostile spotting Ezio, unlike the poison blade where you had to get into melee range. Furthermore, any hostiles around will be distracted as they watch the victim's throes, allowing you to slip by. Contrast with the Hidden Gun, which almost certainly will give Ezio away, and the crossbow which is also likely to be detected. Also, it's always a One-Hit Kill, even on Brutes/Almogavars and Praetorian Guards/Janissaries.
The game's counter mechanic was greatly buffed from the previous games, reducing difficult fights to spamming the counter button, with no regard to timing. In addition, the game's "kick" option while fighting reducing even the toughest enemies to whimpering babies.
Assassin's Creed: Revelations
While Assassin's Creed: Brotherhood expanded II's economic redevelopment system to the entire city of Rome, it was given some reasonable limitations in the way of requiring you to seize control of an area from the Borgias by destroying designated towers before allowing you to renovate the buildings there. Certain towers were inaccessible until later parts of the game, preventing players from 100% renovation too soon. However, the system was taken to absurd levels in Revelations with the city of Constantinople, which had basically the same setup as Rome and none of the limitations, since all of the Templar Dens are accessible almost right away. There was the very minor addition of increasing notoriety every time you renovate a building, which is very easily sidestepped. It is entirely possible to have the city almost entirely rebuilt as early as Sequence 3, making the economy an even bigger joke than it usually is in AC games. This also allows access to the vast majority of the city and side content very early on in the story, unlike previous installments that utilized gating.
Piri Reis' store (and, subsequently, the in-game economy) can be exploited to great effect as soon as you meet him. He will sell bombs to Ezio at a 15% discount, but Ezio can sell these same bombs back to him at full price. With a few minutes of grinding, you can easily make enough money to never have to worry about running short for the rest of the game.
Ishak Pasha's armor set (think Iron Man with a hood) can be acquired as early as Sequence 5. Collecting 25 Animus data fragments will unlock the ability to purchase a map revealing every memoir page location (there are 10 of them) throughout the city, and after collecting them, a final tomb to complete. The armor is one of two unbreakable sets in the game (along with the Master Assassin Armor, which is usually acquired much, much later) and causes guards to freak out and run away when they see Ezio.