Biggest concern is cracking/breaking/splitting the handle material It's also a bear to try to figure out how to secure the overall knife while peening the tang....without wrecking another portion or the knife. There's a lot of things to take into consideration before deciding to peen a tang..... how do I hold the knife when peening? Did I use a handle material that will hold up with breaking/splitting/cracking, etc? How far can I peen without wrecking something? Will the tang material hold up to peening without breaking? Etc, etc, ect.
In reality, it's a far simpler task to thread the tang, and use a "butt nut"....or thread a buttcap.