The main issue probably is the lack of space on the N1.
You need to repartition the internal flash, because ICS requires more space. That can scare some people. And that may mean moving apps to the sdcard. I use S2E with an ext4 partition on the external micro-sd.
And the N1 doesn't have things like the near field sensor.
But once the internal flash is repartitioned, CM9 works just fine on the N1. The inofficial builds just don't have the official CyanogenMod blessings.
And there are also inofficial JB/CM10 ROMs, but they are still considered alpha.