Never understood why helmets in CRPGs did stuff like offer a bonus to saves or whatever. Helmets also should not just cancel Critical Hits. If you are hit in the head with a hammer, a helmet can help, but it is still more of an effective hit.
I suggest leather caps, if you are even going to use helmets as gear in BG3 should provide Resistance to Slashing or piercing damage on Critical Hits only, and only applied to the Critical Hit Dice. Metal helmets should provide the same but for all weapon types. Magic Critical Hits would be dealt as normal.
In other words, Kaedyn is wearing a Leather cap. He is Critically Hit with a longsword. Normally, without cap, damage would be 2d8+2. Enemy rolls a 3 and a 5. So, without cap, damage would be 10. But, cap cuts the second die only in half. Thus, 3 and 2 for a total of 7 damage. Still more damage, just reduced.
Likewise, Vexir is wearing a metal helmet and is hit with a hammer. Crit. 2d8+2 for damage with a normal Crit. However, helmet halves the second die even with a hammer. So, same damage results.