So an interesting tid-bit from the Rubrics datasheet that came out yesterday. The "All is Dust" rule:
Add 1 to the saving throws for Rubric Marines if the attack has a Damage Characteristic of 1. In addition, the -1 modifier for moving and shooting with a Heavy Weapon does not apply to Rubric Marines.
This rule basically makes Rubric marines have a 2+ save and effectively ignore any AP of 1 (reverting them back to a normal 3+ save) against damage 1 weapons (so any small arms).
NOW - if a scarab terminator had this rule (which they may well do as they are Rubrics too), unless there is a "1 always fails" rule, they would be immune to small arms fire, unable to roll lower than a 2 on the dice.
The Heavy Weapons thing is also basically relentless in all but name (though we don't know if firing a heavy weapon still prevents charges, so that's an interesting change if so).
It's an interesting rule, as it could quite easily be ported over to any other terminator with a different name to give them the same small arms fire protection.