Maybe the odds of it doing so are smaller, but if you think a broken nvidia.ko can't screw up anyone's data I think you're simply mistaken.

Say Nvidia's driver has a use-after-free bug: it kmalloc()s a buffer, kfree()s it, then a filesystem kmalloc()s something and gets allocated the same buffer. If nvidia.ko then decides it still wants to use that buffer and writes something into it...kablooie.

Unless you start running some microkernel-ish thing with drivers each running in their own distinct address spaces, you're going to have a hard time avoiding this possibility.

You could be "screwing up" someone's data if an in-tree filesystem breaks. If you read the supplementary blog posts, you would have seen the following:

http://lwn.net/Articles/437284/

Nearly all in-tree filesystems can fail in the same way described there. ZFS cannot. That being said, no filesystem is a replacement for backups. This applies whether you use ZFS or not. If you care about your data, you should have backups.

Well, to be honest, all filesystems that are currently in the kernel tree started out as being maintained outside the tree. Inclusion into the kernel is normally one of the end goals. It's part of the standard progression - 1) rapid development outside of the tree, 2) once the filesystem is stable, it negotiates for inclusion, 3) inclusion into the kernel, 4) maintenance / updates as part of the main kernel development process.

ZoL isn't even trying to get included into the kernel, so it's a bit of an odd duck here.