It is rare I have found a tree with significant enought decay to cause structural concern that cannot be noted using simple knowledge of trees and trained observations. To me where the resistograph can be very valuable is if you want to leave some trees with a certain amount of decay. Use the resistograph to document the amount of decay along with detailed notes about why the tree is deemed safe. This is done so that you have records that you have used the best information on hand and followed industry standards in determining the tree was structurally sound so that if it does fail, it would be considered an "unpredictable" event. On the other hand, if you need to justify removal of a hazardous tree, proving with the resistograph that the remaining walls in the tree are too thin to support long-term safety.