I run a spicer, I go 20mph everywhere I go, and I have dumped the clutch in rocks bigger than my front tire. I run 22x11-8 atv tires and never had a problem, not even a micro crack in the case, but I have a open diff. Most people use peerless, because the bull gear pin is bigger(to get more metal to weld to for locking), they have more fins on the case(to help hold the case together since locking them stresses the case more), peerless took more time and care to build the tranny, and that's about all I know why they use them. I hope I helped you out. I never had a peerless myself, but that's what I get out of watching youtube vids.
Also, I think most are grease packed, but Doc suggest if you are going to go faster to convert it to oil filled.