"... just like any dog or other animal, our pet birds don't really realize they are being limited by the harness"
On this point alone I think that it's worthwhile to keep in mind that birds have a very strong feathers=survival Instinct. A birds preening skill seems to kick in immediately after birth demonstrating how important feathers are in their lives (not just chickens but I've even seen baby birds with downy feathers preen themselves). For this reason we shouldn't assume that their psyche with regards to feather disruption can be compared to a human toddler or any other mammal that is being restrained. We've all seen how our birds, upon becoming relaxed and at ease, will immediately commence preening themselves. In addition, mutual preening is an affiliative behavior - done between individuals in order to strengthen their pair bond. It is a sign of how important properly groomed feathers, as well as removing imperfections on those feathers, are to birds. How many of us have had birds preen us and, sometimes painfully, try to remove imperfections on our own bodies? - this is that very same instinct!
The point being is that we do not downplay how stressed a bird can be when some object is causing feather disruption. And that it may cause that animal greater stress because their experience of it is not comparable to a human toddler in a harness or a dog on a leash.
I first read about extensively in Dr. Lafeber's book "Let's Celebrate pet birds."
But for further insight, here are some quotes I grabbed from Wikipedia...
"Displaced feathers can cause birds considerable trouble; such feathers might become damaged, could interrupt the smooth flow of air over a flying bird, or might allow the bird's body heat to escape. "
"Because feathers are critical to a bird's survival — contributing as they do to insulation, waterproofing and aerodynamic flight — birds spend a great deal of time maintaining them."
Sorry I can't comment on any particular harness as I don't use them.