Yeah there are a few times when you can one cut a tree in one pass, however if a tree is that large that you can't actually move it by hand, then you want to be careful unless it is just straight on the ground, any angle is a potential for a snap. But in general the whole idea of cutting very large wood is just silly, what are you using it for, even a medium sized tree, say 8" across can weigh 350 lbs with dense wet wood, you really want to try fooling with that in an emergency situation.
I have spent the last few weekends hauling wood from 8-12" thick, usually you only need to carry them maybe 50 to 100 feet to load them on to a truck, however even when cut to length (8-12 feet), they are very heavy, you stumble with a log on your back of several hundred pounds and it isn't going to be a happy day. For shelter building and fire burning you don't need anything beyond 2-4", even if for some reason you have to cut large wood, you can do this easily with a smaller folding saw using notch relief, which you want to be doing anyway for directional control.
The only real use of those saws is that they allow you to store a saw pretty much anywhere you can basically carry them in your pocket. Just realize what they are and what they are not. I can carry a piece of sandpaper in my pocket as well, but it isn't the same as a set of files, hones and a strop.
-Cliff