There have been several studies that indicate that scents and scent blockers really don't do much (if anything) to dull an animal's reactions.
In my limited experience, I've seen that deer in particular don't pay much attention to anything but movement. I suppose that if they have been spooked or harassed they will get more skittish to scent, but I've been walking along a trail, wit the wind at my back and walked up on deer.
If they didn't bolt right away, lots of times they would stay there trying to figure out what I was, as long as I stood perfectly still. I've had them snorting and stamping trying to make me move.
With respect to ghillie suits, it is my understanding that they were designed to camouflage ghillies who were attempting to find and catch poachers. Humans see things in the woods very differently than most animals.