From the one time I cosplayed as R.J. MacReady, the three main problems I had were as followed:
1. Heat - I was dressed in a gray T-shirt over a thermal shirt, so I was sweating a lot. I had sunglasses on with felt pieces on the sides to mimic glaicer glasses, which didn't allow for the heat to escape the area around my eyes, so it fogged up the lenses.
2. Weight - Weight was another issue for me, as I was lugging around a flamethrower pack that probably weighed as much as the real thing. This one may vary between each cosplayer, depending on what they've got on and its weight.
3. Maneuverability - Going forward was easy, but going backwards or turn around was a bit of a pain, as I would often either be bumping into someone or something by accident or still being in the way of someone else due to the flamethrower pack.