I’ve never lost a single grafted chestnut. The Dunston grafts were said to be fragile in my zone, but I think the problem may lie in their excessive precocity. I simply removed flowers the first few years until trees were well established.
In my nursery (Z6) I always have a few grafted chestnuts in the mix, though mostly Chinese or ChineseXAmerican types these days. My customers are looking for nuts- not timber.
I purchase trees from Nolin River.