It's not true
. . .
but what is true is that they are flightier, like some wilder colubrids. Since many first generation crosses are coming from wild collected SD locales, we tend to get that "wild" feeling from them that makes them less domesticated. In some aspects, the first generation crosses from any species tend to be a bit harder to work with, but lets refrain from the term nasty by all means.
My 3rd generation crossed 50% SD F2 Tiger is a big baby. She's only 8 ft. Is 4 years old as of this month, and I've raised her from 80 grams. My 8 year old first generation cross 50% SD Tiger male... he has his moments of being difficult, but he's normally quite calm for a first generation cross.
Jampeas have been bred in captivity for probably the longest amount of time, and thus tend to give more docile genes in the flow. All my Jampea crosses are wonderful and calm.