THere have been reports of Mack snows not working out the way most geckos work out with temp sexing. I've been breeding Mack snows for at least 5 years and have had no problems with this. I recall a thread (do a search) where people were reporting on their results and no one had been having problems recently.
Ironically, just last night I was reminded of this issue (per leopardgeckowiki) when I was going through hatchling tubs to find that at least three Mack babies were all showing male, and all supposed to be female. I also have a female that was incubated for male. The only Giant Snow Diablo Blanco I hatched this year is also male, incubated for female. I've heard of inconsistencies with other breeders as well, but it doesn't seem to happen for all of us.