The recent Hive Fleet adaptations preview from the community page (
here), has shown Hive Fleet Jormungandr's adaptation as such:
"A unit with this adaptation (other than units that can FLY) always has the benefit of cover for the purposes of shooting attacks. If the unit Advances or charges, however, it loses the benefit of this adaptation until the start of your next movement phase."
The
Iron Warriors' legion trait (and similarly the
Imperial/Crimson Fists' chapter tactic) states that "Enemy units attacked by units with this trait do not gain any bonus to their saving throws for being in cover."
Now, I'm wondering if the Iron Warriors' trait completely negates Jormungandr's adaptation - it could have been written to say "units with this adaptation gain +1 to armour saves unless they have advanced or charged" or something similar, but it explicitly states that the bonus counts as "the benefit of cover". Despite playing Iron Warriors I can't say I'm too fussed either way, but it would be interesting to see one of these traits completely hose another.