It allows the child to sit more upright and even wiggle around a little, and some even will hold two kids. Lighter and smaller means less stability but less effort. The sled can tip over if the child sticks his arms out and reaches for something or you go over rough terrain. We have two children. Our first took to our pulk right away, our second would not go near it.
Every kid is different.
Some parents are totally against pulks because, as with backpacks, some kids are slow to move onto their own skis if they can get a free ride. After watching some parents dragging their kids in from a tour holding on the end of one their ski poles, I decided to always carry a short rope with a handle tied to my belt pack. This allows me to tow a child when he has had enough. Make sure that the rope is plenty long enough to keep your child well away from the end of your poles when you are poling, otherwise you may stab him with end of your pole. This works well when you have two children and the older one wants to go on a longer ski than the younger one can handle easily.