Auto critting is too powerful. Especially when we get higher level sneak attacks or smites. But breaking nonmagical shields and armor could be cool, it’s too fiddly for tabletop but it could work in a game. There’s a slight downside of less loot but not a big deal by the time this matters. You could maybe make adamantine weapons auto-crit constructs as if they are objects.
Another simple solution would be adamantine weapons are +1 and also increase your crit range to 19-20.
