In 5e ... adamantine weapons, bolts or arrows always do critical hits.
Only to objects. Still, giving the major benefit of adamantine armor to generic metal helmets pretty much ruins adamantine in BG3.
Yeah, I fear their design philosophy creates more problems than it fixes. If protection from critical hits from the common helmet is extended to enemies, there will be little point in having a Champion subclass for the Fighter. Further, I don't want my party immune to critical hits, and if this property is tied to all helmets I won't be able to use any of the magical helmets I find. This property should be restricted to Adamantine Armor, so there is something cool about it, no homebrew needed.
