FFS is not supported by the delivered u-boot. Not a surprise, nor NetBSDs fault of course. I'm working around it by putting the kernel on the on-board boot partition so far, and the kernel can read the root BSD filesystem.
There's people on the FreeBSD side of the house that are working on getting a FFS into u-boot. We had an implementation a few years ago, but it was rejected for bogus reasons. There's been on and off interest in reviving it. It doesn't look too hard, and the political environment is much better now than it was the first time. Too bad the interfaces have changed radically in the mean time.
If people want to work together on this, drop me a line.