If your PSU fan is facing upwards and you plan to mount your HUE+ on top of your PSU then you'll be blocking the PSU intake with it.
You can mount your PSU fan facing downwards just fine since Be Quiet! case has bottom intake holes for PSU.
As far as risks go with seating anything on top of your PSU, that i'm unsure with. Just to be on the safe side, i wouldn't seat anything on top of the PSU.
I mounted my HUE+ into my 3.5" drive bay with the Corsair HDD bracket that also supports 2.5" drives.