Disable ShadowPlay from GeForce Experience before entering the game!
Introduction
If you encounter a crash on startup, then you'll need to disable ShadowPlay from GeForce Experience before entering the game!
How to?
Open GeForce Experience, click on ShadowPlay in the top right corner, then click on the "switch" thingy, until all green lights are off. Exit both ShadowPlay and GeForce Experience.
The Result
Voilà! You can now enjoy playing Assassin's Creed Unity again!