When water is gradually diluted to hyposaline conditions, the osmotic pressure on fish is reduced. It takes less energy for them to function, breathe, etc. It has been observed that fish badly infected with ich suddenly have more energy and vitality when they are put into hyposaline conditions, and this greatly aids them in surviving the outbreak.
Why do you feel hypo is hard on fish? I have never seen any evidence of this in the fish that I've treated, nor have I heard this from others who have done many more of these treatments than I.
Copper is a toxin. When fish are treated with copper, great care is required to not kill the fish along with the ich. Copper can cause organ damage in some fish and long periods of exposure are harmful. I liken copper to "chemotherapy" for fish, although that's an exaggeration on my part.
Hyposalinity treatment, on the other hard, doesn't appear to subject fish to stress. The process of diluting the water is stressful on the fishkeeper, though, no argument there. And I suppose if not done carefully, fish can kinda freak out from all the activity. When I do water changes to drop to hypo, I siphon water out quickly, and then I drip the replacement water in over a few hours so that there's as little "action" as possible around the tank, to reduce stress.