Elemental/shadow sowrds deal split normal. This mean half the bonus won't go to the important part of the sword but to the neutral one. This may make said weapon better against all enemies, but not specifically to those it was designed to kill. 50% of the bonus is 'wasted' on the normal damage, not the elemental/shadow one.
On the other hand, Gus have a single damage, so any bonus will greatly increas it's power against it's inteded tarets.
For example, a Glacius with damage bonus will see a 7% increase in effectiveness against undeads, while a Polaris will go up to 9-10%.
Might not be perfectly balanced and calculated, but you got the idea.
I know piercing swords and antigua upgrades are special cases for this rule.
Unless I missed something, Shadowsun doesn't appear in your post at all. Please help me out here.