"Risk of Psychological Difficulties Among Children Raised by Custodial Grandparents - Similar to other children in kinship care arrangements, custodial grandchildren are reported by their caregivers to have higher levels of behavioral and emotional disturbances than children in the overall U.S. population."link
It's not appropriate to compare the typical grandparent adoption in the ghetto to the grandparent adoption in, dare I say it, in financially stable white Christian families.
Nothing against the valiant ghetto grandmoms who rescue their grandchildren from their druggie kids—they deserve credit (unless the problem is that they had illegitimate kids when they were 15 and the cycle is repeating). But the cultures, as of now have been vastly different.
Unfortunately at this present time, the rate of illegitimacy among blacks is three times as high as among whites, and the rates of teen pregnancy and drug abuse are also skewed upward for blacks.
It's not pleasant to hear, but unless the studies you cite also break down the financial status of the adoptive couple, whether the grandparents are married and both the biological relatives of the child, and whether the surrounding community is on the low end or the high end of illegitimacy and single parenthood and social disorder, simple math lumps too many culturally disadvantaged cases against the so-called "average."