Both these statements can be true. Their drivers can be excellent but try to do anything with your Linux machine after the Nvidia drivers are installed and you run into trouble.
Ubuntu release upgrades used to be impossible w/ Nvidia drivers. You'd get into a situation where you'd boot into a text-only console, but because nvidia didn't do kms you'd get 40x25 with the first few characters off the screen.
Nvidia has excellent Linux drivers.