I love carni's.
If you took a scale from one to ten, and rated honey, you would have Italians on one end (lets say 10 being the best) and Russians on the other end (That being 1).
Then take another scale from one to ten for mite resistance, you would have italins at 1 being the worst, but you have Russians at the other end being a ten
Carni's to me are not as good as Italians with honey production, say an 8. And they are not as good as Russians with mite resistance, but they are an 8. They may not be better than either of the other two bees in their own top categories, but overall, they are close enough on the items I suggested, to make them a very good overall bees. Good in both honey and mite resistance. And my best bees are hybrids between the Russian and carni.
One thing to keep in mind. Labels for bees should be used for generalized comments. There are good Russian breeders and less than adequate. The same goes for carni, Italian or anything else. Quality queens still comes down to a quality breeding program. You can but 10 different Russians from 10 places, and see drastic differences.
So pick out a good stock you would like to to try, then spend 5 times the research seeking a breeder. Ask questions.