I'm afraid we are a little outdated here. I can only dream of what it would have been like with today's NVDs.
Double and triple canopy lends protection from overhead detection. Working under it is like working in a huge warehouse since there is little if any undergrowth. These were favorite hiding places for the NVA while moving down the trail. OTOH the terrain around the edges of such jungle was extremely dense and HOT!!
Noise and light are self explanitory. Anything out of the ordinary can draw attention.
Grabing bushes and limbs for handholds is like sending up a flare.
I smoked "Yard" tobacco.
My .05 cents.