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.
Hmmm, my argument is that in 5e adamantine weapons do auto crit, and the fact is you only get one adamantine weapon, but I see your point... but I want the adamantine weapon to worthy of the risk/fight.
Maybe a combo of your ideas
it's a +1 (maybe a +2) weapon with crit on a 19-20 AND auto crits constructs
though the more I think about it I think each weapon should have a special feature of its own (and they should also include a great weapon in the selection) to make the decision that much harder.
So for example all Adamantine weapons are +1 with crit on a 19 or a 20... then:
Mace- has reeling for 3 turns (I kinda feel like the reeling ability is okay just needs a longer effect)
Longsword- Chance to break nonmagical shields and armor
Scimitar- Chance to disarm (after all it a finesse weapon)
Great Hammer- (as the great weapon addtion) auto crits constructs
though that might be getting too complicated.
All I know is that risk vs reward for the adamantine weapons and armor isn't on par with some other items in game.
(Example the Everburn Blade)
