You will have to work to find the usable trees on trails in NH White Mountains. I did a Pemi loop and found it a challenge to find trees that I didn't have to bloody myself bushwhacking to. I made many forays off trail to find spots 200 feet from the trail and often returned to the trail frustrated. Also had to descend a mile down a trail to find a spot where the trees were hangable. Above 3000 feet the trees are scrubby and closely packed in with lots of deadwood. As many others have said forget about the ridges.
You can definitely hang a hammock in the whites as there are plenty of trees, but plan carefully. You may need to descend to find trees that aren't in an impenetrable scrubby thicket.