The upcoming Patch 4 for Assassin's Creed Unity is set to, in Ubisoft's own words, "rectify most of the remaining issues our players have been reporting". Thus, finally, it will be ready for release. Just kidding. It came out a month ago.
One "top priority" issue is that the game can crash after a save is loaded, which, Ubisoft says, they're "devoting a lot of attention" to. For now, there's a workaround: "try taking your system offline and reload your save, you should be able to play offline."
You can see a list of what Ubi has identified as the biggest issues over at the AC:U live updates blog. Patch 4 doesn't have a set release date yet, but a more detailed preview is due later this week.