The latest game consoles aren't even a year old, but developers are already bumping into the limits of the hardware. Loads of games run at sub-1080p resolutions, especially on the Xbone, which has less graphics oomph than its PlayStation counterpart.
Assassin's Creed Unity is another casualty; senior producer Vincent Pontbriand told Videogamer the title will be limited to 1600x900 resolution and 30 frames per second on both consoles "to avoid all the debates and stuff." GPU horsepower doesn't appear to be the most important limiting factor in this case, though. According to Pontbriand, Unity is more bottlenecked by the CPU, which "has to process the AI, the number of NPCs we have on screen, all these systems running in parallel."
Developer Ubisoft Montreal apparently didn't expect the CPU limitation. The team anticipated "a tenfold improvement over everything AI-wise," Pontbriand said, but they were "quickly bottlenecked." He describes the limitation as "frustrating" and says the game could be running at 100 FPS if it was "just graphics." Seems like this generation of consoles has enough pixel-pushing prowess for prettier visuals but not enough CPU power for richer gameplay.
The Xbone and PS4 are both based on custom silicon with eight x86-compatible CPU cores. That might sound potent, but those cores are derived from AMD's low-power Jaguar architecture, which is designed for mobile devices like cheap tablets and notebooks. Intel's eight-core Core i7-5960X processor is a whole other class of CPU. So is the dual-core Pentium G3258 Anniversary Edition, for that matter.
Assassin's Creed Unity is coming to the PC, and that version shouldn't be locked to a specific resolution and frame rate. However, Pontbriand doesn't say whether the AI and other systems will exploit the more powerful CPUs available on typical PCs. That outcome seems unlikely, especially since Ubisoft is optimistic about releasing Unity simultaneously for the PC and consoles.