I don't want to go into technical issues too much, because ultimately they're not Unity's most significant problem. As for what that is, well there are several contenders. After the nautical brilliance of Black Flag, a return to a more traditional Assassin's Creed game was always going to be a difficult sell. Ubisoft assured us, however, that Unity would see the game's core systems reworked to address the series' endemic problems. Namely these are unpredictable free-running, boring combat, and nonexistent stealth.
It's true that Unity makes some changes to these systems, but none of the alterations have the desired effect. Free running has been slightly improved by distinguishing in the controls between free-running upward and free-running downward. Now it's much easier to get down off a building without breaking both of your characters legs. Simply hold the "free run down" key, and Arno will skip nimbly down the side of whatever structure he's standing atop. The consequence of this is roof-top free running is mostly without issue. On the other hand, the system really struggles with the new interior areas that Unity shows off so proudly. Arno gets stuck on walls, furniture, staircases. Imagine a hippo trying to survive in a London studio flat, and you get some idea of how Arno handles indoor life.
Combat is even worse. Ubisoft have dispensed with the simple counter-based system seen in previous games and replaced it with...a more complicated counter system. Now instead of counters resulting in an instant kill, they merely give you the opportunity to attack your opponent. But the attacks are still based on the same sluggish, choppy manoevres we've seen before, too concerned with flourish rather than speed. Compared to the recently released Shadow of Mordor, which sported combat so slick it could con Derren Brown, Unity is like watching a pair of children mimic a swordfight with sticks.
Even the new stealth mechanics are weirdly thought-out. Ubisoft have finally figured out that, when making a game about assassins, giving the player the ability to crouch, move quietly and hide behind walls is probably a good idea. But the button you use to stick to cover is the same button you use to climb things (while also pressing shift). Moreover, they've removed the incredibly useful "whistle" ability seen in Black Flag, so there's no way to draw enemies closer to you to dispatch them in a quiet location. It's unfortunate because the "sandbox" Assassinations that are one of Unity's main new features carry a lot of potential, but the stealth systems simply aren't tight enough to make the best of them.
So the half-baked systemic changes are one problem. Another is how Ubisoft fail to capitalise on their beautiful Parisian playground. They choose slather this wonderfully created city with an abundance of "content", like a spice addict drenching a Michelin-star dinner in hot sauce and obliterating the delicately balanced flavours. This is a problem we recognised in Watch Dogs, but it has reached a new level of absurdity in Unity. You can barely see the map for the amount of icons cluttered on it, the innumerable collectibles, the three different types of treasure chests dotted around the map (some of which can only be unlocked using a companion mobile app), shops, safehouses, points of interest. Not to mention the missions and side-missions.
For what it's worth, the side missions are pretty good. Best of the bunch are the "Enigma" missions, which hand you a sequence of riddles to solve by visiting specific locations in the city that which riddles cryptically direct you to. The murder mystery missions are less successful. These involve deducing the correct suspect in a crime by putting together a sequence of clues - often located in different parts of the city. They're fun in theory, but in practice they're very simplistic. In one particularly insipid example "Hot Chocolate To Die For", there was only one suspect who I could accuse. Less Poirot, more Scooby Doo. Assassination missions are as good as they ever were, with Ubisoft spinning them out in an impressive number of ways.
But the real issue is there's nothing that binds it all together. In Black Flag everything fed into upgrading your ship so you could take on larger quarry for bigger prizes. But Unity has no such dynamism to lend its city purpose. The best Ubi can do is give you money to unlock better equipment, which you receive anyway during the main quest. Remember, this is a game about the French Revolution. Why am I not swooping down from on high to rescue French nobles from the guillotine like a weaponised Scarlet Pimpernel? Or perhaps helping the revolutionaries take over the city by raiding the power-bases of the Establishment? There's so much potential here for a fluid, unpredictable, endlessly creative open world game, and yet Revolutionary Paris is little more than a theme park filled with pre-designated rides.
One last aspect of the game I haven't yet mentioned - Unity's cooperative mode. To put it bluntly, it's fine. Between two to four players partake in multi-staged Assassination missions that see you leaping across the city together. Again though, Unity's unreliable control system means trying to sneak four assassins past a bunch of guards is essentially impossible, and unlike other coop stealth games like Splinter Cell: Chaos Theory, Unity doesn't really encourage players to work together [WARNING: IRONY LEVELS CRITICAL]. In the missions I played, what usually happened was players far more capable than me ended up rushing off ahead and doing all the work, while I was still messing around trying to get Arno to climb the right way across a bloody rooftop.
Ultimately Unity comes off as a busier rerun of Assassin's Creed II, which given that Ezio's original adventure is five years old now is deeply disappointing. The story kept me engaged to a point, and leaping about the city is as fun as it ever was, but everything else lies somewhere between slightly broken or simply outdated. All this combined with the technical hindrances means that Assassin's Creed Unity is a long way from the revolution we were promised.