ok, it it the fact that it's a nylon harness or the style? I'm hoping to get a harness for my mini in the next year & I know nothing about them at this point.
When you get a cheap harness it does not have the adjustments needed to get a proper fit. You are pretty much stuck with it as is. Besides the areas of pressure points are narrow and do not distribute the weight properly to be comfortable.
I was helping give a driving lesson to some people who brought a pony with an ill fitting cheap harness. I showed them the rubbed marks on the ponies back where it had made the hair white. They said no that the pony was spotted. I put on the back band and showed them how it was the exact same size and square on both sides where it had been rubbing the pony. Not sure if they finally got it or not but it was not a spotted pony but rubbed and I am sure it had been painful.