Unity was basically their first attempt to do what Origins and Odyssey successfully did.
-Overhaul the core mechanics and upgrade system to emphasize multiple playstyles
-Overhaul mission design to eliminate strict fail states and linear missions in favor of simpler objectives that can be completed in multiple ways
-Create a more systemic world to emphasize player driven storytelling
-Create much more content that fleshes out the world and characters on top of maintaining some longevity and in theory, making every area of the map worth it to explore
Thing is, unlike the sequels, Unity had some design decisions that directly contradicted the above.
-Overhaul the core mechanics and upgrade system to emphasize multiple playstyles-->Combat was still slow, monotonous, and designed to be the path of most resistance even when you had a five star weapon and super large health pool thanks to armor, so they sorta pigeon holed you down pursuing stealth when the intent was multiple defined playstyles
-Overhaul mission design to eliminate strict fail states and linear missions in favor of simpler objectives that can be completed in multiple ways-->This is true to some extent but it only applies to how you reach your targets in main assassination missions as you still had to finish them with the hidden blade, each mission also had a clear hitman inspired linear defined path that was the best way to go in spite of the option to do something like start a fight. It was often more compelling to follow that path instead of choosing your own as the game systems didn't support that amount of nuance
-Create a more systemic world to emphasize player driven storytelling --->The game world, while insanely impressively, was still largely within the trappings of older AC games, with very clearly marked factions, a very small pool of crowd events that popped up, etc. which never felt truly systemic like in the Origins/Odyssey games, so each player had largely the same experience
-Create much more content that fleshes out the world and characters on top of maintaining some longevity and in theory, making every area of the map worth it to explore--->Unity did this part well, with the varied co-op missions, bank heists, murder mysteries, and even character related missions that helped flesh out the cast which was sorely needed since the main plot only went surface level when it came to character motivations outside of Arno and Elise, with no one else getting proper screen time.
As it stands, the other two AC games lack those design contradictions and feel much more coherent as a result on top of the games just feeling way better to play thanks to the simplified control scheme and compelling upgrades.
So all in all Unity is a game with a good premise and clear passion that fell short due to several factors. From the troubled development to some odd design oddities like the extremely limited cover system.