Sorry to say, my experience mirrors yours - Getting Linux to work on a NuBus PPC machine is far more trouble than it's worth.

In my experience the main problem has something to do with the kernel being unable to properly interface with the IDE bus - which, interestingly enough, some folks had trouble with in later years when trying to install Linux on the Rev. 1 Blue and White G3s with their dodgy main IDE bus chip.

Matt