Hi Nik,
I did another test, still a bit confused:
When I create a smart Album, using Nitro with the Photo’s-library, It will create also a smart album in Photo’s, showing up in the Nitro folder (with series of letters and the name). However the photo’s selected and visible in Nitro don’t show up in Apple Photo’s. The smart-album stays empty in Apple Photo’s
Actually I think this is fine to me, as it enables me to delete the smart album in Nitro, without loosing the photo’s in the photo’s-library. Not sure whether it is intended though.