I've used a No2 for about 20 years. I love it, but I wouldn't want to take it backpacking. I bought it at a boat jumble in Nottingham for seven pounds and I used it on a boat for five years. Only ever on deck, mind, not in the cabin!
Phil's right, they're all of the above, plus a bit difficult to control because the long steel needle
[*] in the valve expands and contracts with temperature so you can't just set the flame and leave it to simmer. You have to watch it all the time.
[*] Or it might be the tube that it runs in...