What is nouveau?
There is an arguably superior driver supplied by Nvidia. It is faster, has better hardware support, and is arguably more stable (1). However, it ships as a closed source binary blob with a source-based shim layer to integrate it into the kernel.
(1)My wife has 2 Linux desktops with Nvidia GPUS. She ran nouveau by default. However, on both machines, it would cause a kernel oops, and lock the screen up. My wife assumed it was her entire machine locking up, but I realized from the stack traces that were recorded that it was the GPU driver. I then switched the machines to the Nvidia driver, and they have been stable ever since.
I personally always buy hardware from Nvidia, because they provide a binary driver for FreeBSD. They are one of the few companies in the non-server space to provide good packaged drivers for FreeBSD.