I also could not see anything wrong in logs, so ubi is not to blame.
I had similar problem when tried to boot nitdroid from p5. For me it was just a wrong link (to nitdroid folder itself). I had just black screen.
So it seems ubiboot can see the nemo kernel, but execution with given init fails.
Please give us also your ubiboot.conf.
And make sure you extracted the nemo.tar correctly (tar --numeric-owner -xf nemo.tar) on device itself and on correct partition (p4). Unzip the nemo.tar.bz2 on PC before copying to N9 of course.