I have read about this technique, and from what I can recall (can't find the article, though I thought it was by Harry Harrington) it is used mainly on pines. With thinner barked species, you run the risk of crushing the soft phloem layer and causing the phloem to separate from the cambium and/or xylem. The gist of hammering is to thicken the trunk of a tree that is nearly finished/mostly developed. For less developed trees, it would be far safer to put them in the ground or to grow out sacrifice branches.
There are other artificial trunk thickening techniques for thin-barked trees, such as poking with a large needle or making small vertical incisions. These are probably a little safer than hammering, but they are still somewhat drastic measures. With these techniques, you also run the risk of infection. Again, these techniques are typically applied to nearly developed bonsai (usually in cases of reverse taper) where growing vigorously would cause a loss of fine details.