I usually cut my butts completely in half, so they're 2 separate pieces, not really butterflied or spatchcocked. If they're partially frozen, gliding along the bone instead of into it can be challenging. I see no downsides if you're "pulling" all of it and they definitely cook faster including getting through the stall faster. If you want to slice some (not just pull it) cutting it the "long" ways gives smaller pieces. So sometimes I cut it the short way, though occasionally the bone interferes with that.
All that said I just did a butt yesterday and since I'd planned to start early and was fine eating late, I put the whole thing in. At 9pm, I was at 189F internal (just coming out of the stall) and I threw in the towel. (I'd run out of charcoal in my "labyrinth" as well.) So I brought it in, put it in the wife's oven at 375F for 30 minutes (to 210F internal) and it came out perfect. Half inch thick smoke ring all around. Still, I regretted not halving it up front...it was fully defrosted.