Just flashed a new MicroSD with current Raspbian light and installed the rainbow-stuf via \curl -sS https://get.pimoroni.com/unicornhat | bash - problem still exists.
Can you try the rotation setting (in Python for the unicorn hat) to rotate 180 degrees?
My simple idea that might help point to hardware or software depending on whether the unlit lights will now light.
finally found the time to dig into this problem again. funny thing, i have repurposed the zero w with the phat dac in the meantime - so i’m using another one for the unicorn phat - and everything works fine. problem solved, maybe something was wrong with my soldering on the pi side. thanks for your help everone!