I don't care what people use. I've mostly always used a leather collar, unless doing leg and suit work. I know some people have a strong opinion one way or another on them, myself, I don't think it makes that much difference over all. In some cases sure, but generally not.
with harnesses I do notice a lot of handlers don't have much control, so when their dog goes to take the sleeve or pillow to the ground and munch or thrash, i think a collar is much more handy.
If a harness is choking a dog, it isn't fitting right and needs to be adjusted. I haven't seen any adjustable x-back harnesses and every leather one I've seen is. Xback also gives zero lateral contol, it just folds over the back. I have one and use it to have my dogs pulling things. I'd never use it for bitework and if I was the helper I wouldn't allow it to be used on a dog I was working.
they may be able to handle some stress, but don't kid yourself it is NOT the same. There is pressure at the start, and then once momentum is generated, the forces go way down and it's a relatively constant application of force and at the start there is often help in the form of pushing from the handler either by feet or skis, and if it's a sled, the load is spread out over multiple dogs.
Bitework is very jerky. Lots of maximal load, back down to nothing, back to maximal. There is no loss of tension by reduced friction and gained momentum, it's stretch to load, stop, and back down. Very different forces, very different effects.
slow steady applications of load tend to be far less destructive than sharply applied ones that are repeated.
the xback might hold up for a while, I'd never trust it over a well made leather harness with heavy duty rivets.