Rainbow Six: Patriots was announced in November 2011 and should have been out along 2013. Logically, this Rainbow Six chapter has suffered a lot of issues and two different sources have detailed what has happened with this game.
The Anvil Engine was the cradle for all the troubles. This engine never had been used for a FPS and provoked issues in the overall stability and the frame rate instability was unsalvageable. The team was formed by 120 people. These people were not able to create the game quick enough and dedicated too much time and resources to create the original proof of concept (the part that we saw it).
Level designs were done and the story was written. Finally, Patriots was moved to next-gen platforms.
Patriots haas have (at least) five creative directors. The team working on Patriots right now is almost completely different than the team that started the project. Ubisoft Montreal declines to speak about Patriots, perhaps the game is no longer in development.