We wouldn't normally publish individual workarounds for specific game bugs, but the launch of Assassin's Creed: Unity has been ugly enough to warrant an exception. And this isn't actually for a specific bug, nor is it guaranteed to fix anything, but if you're having problems with the game crashing at the main menu, Ubisoft recommends you delete all of your game contacts. Yes, you read that right.
The latest addition to the Assassin's Creed: Unity live update site says Ubisoft has discovered "one of the issues that may cause the game to crash at the main menu after pressing the 'continue' button." A fix is in the works, although there's no ETA at this point, and in the meantime, if you haven't been affected by the bug, there are two things you can do to keep it from making you miserable.
Step one: Do not add any "in-game recent player" as a game contact.
Step two: If you already have any game contacts, ditch them. All of them. Don't waste time apologizing. Do it now. Quickie instructions on doing so: My Brotherhood > My Contacts > Game Contacts (using RB/R1) > Focus on a Contact (LS) > More Actions (Y/Triangle) > Remove from contacts.
Getting rid of all your contacts doesn't seem like an ideal solution for a game that was billed so heavily as a multiplayer experience, but better playing alone than not at all, I suppose. If you experience crashes at the main menu even after cutting all your contacts loose, then Ubisoft asks that you please file a ticket at the official support site.