I had a 6" Vermeer 606 PTO
chipper. It was about the heaviest duty model I could find, it had a 6x6 opening and was hydraulic feed. A lot of times if I didn't trim the larger branches or small trees just perfectly, a branch would get hung up in the feed and I'd have to back it out.
I think Wallenstein's design with the 6x12 opening for the 6"
chipper is a better design, although I have no personal experience with it, I could absolutely guarantee that I would have had a lot less hangups if the Vermeer was similar.
I sold mine, it ended up being way more work than it was worth. Now I just grapple the brush into a pile in the woods, or burn it.