I have shot several deer and antelope with a muzzy all with powerbelts and for the most part they have all worked really well.
I did have a bad experience on my last antelope though. I hit a little forward and hit front shoulder at 70 yards and it didn't get past the shoulder. You could see the bullet hole on his shoulder as he was standing out there at a few hundred yards clear as day. I searched for that buck for a week and never found him. I had shot an antelope buck at 50 yards or so through both shoulders with the same bullet a couple years before so, I'm not sure which experience was the fluke?
I personally think the powerbelts are too soft and when I draw another muzzy tag I will be looking for something a little more solid. Maybe the no excuses or Hornadys great plains bullet? I think an all lead bullet should still be able to punch through an antelope shoulder at 70 yards and one bad experience is enough to steer me away from them. I definitely wouldn't have the confidence to use them on an elk. Heck, a big bulls ribs are almost as thick as an antelope shoulder bone! I always did like how easy they loaded and accurately they shot, but just not worth it.
That's just my personal experience with them, so no hard feelings if you use them and like them. They killed 5 out of the 6 animals I shot them with. That just wasn't good enough for me.
Personally I think Idaho should allow copper plated bullets. They don't increase a person's range, but they sure do help a bullet hold together when it impacts. "Technology" like that only increases recoveries without harming the tradition of the sport.