I first tried Nirvana 15 years ago. I had to switch from the canadian seedbank I was using. I picked Aurora Indica, It was decent for back in the day, they seemed ok.. But it was really hit or miss with the genetics.
I thought it was pretty obvious.. Serious seeds was doing AK47, then Nirvana comes up with AK48. I mean come on really?.. KC Brains Mango, was changed to Nirvana Papaya.. Knockoffs, plain and simple.
If i wanted a specific cultivar like say Chemdog, I would go with a company like Dynafem. The fem'ed seed is a self polinated female, so the genetic variation wont be that much. So you dont have to look, like with regular seeds. I'm not sure how Nirvana does it, but I'm sure the just buy a standard pack from the breeder, then finds a P1 and P2 from that pack/s/ and sells the knockoffs F1's. or they may go another round and F2.. Who knows?.
Something like that..