What could help make armies like this tougher, and makes fluff sense for several other armies, is an Immunity USR giving them advantages against poison weapons.
Where this makes most sense is Necrons and Nurgle. This actually came up when talking about Necrons with a friend and how they have virtually no chance to beat the new DE codex.
This could work 2 ways:
1- It reduces the roll required for poison to work
2- It is a save versus poison weapons only
1- This method would say subtract 1 or 2 from the to wound result of a poison weapon, so say a poison 2+ would become a poison 3+ or 4+. 6 would always work, so either a poison 6+ weapon would not be effected or it would require another roll of say a 2+ or 3+ on top of the 6. The issue with this is that it complicates the game a little more than option 2 and is indiscriminate to the "toughness" of the army. I believe necrons to be more immune to poison weapons than anything seeing as they are made of metal, but under this they would be the same as anyone.
2- This would give a save like FNP versus Poison weapons. Where this is better than option 1 is that you can scale it better. Say the Plague marines get a 4+ save versus poison where as Necrons get a 3+ save. The issue is though that with cover saves and Feel No Pain is another save really needed or will it ever come up?
So with the issues of number 2 and the complications of number 1, maybe they should just make up a Army Wide special rule for the Necrons when they drop. I would favour a save like FNP for them, but only versus poison weapons. This would make them tough in the right way, and while it seems to be prejudice versus DE, they still will have a large advantage in Close Combat and immunity helps against Tyranids, SM Sternguard, and Nurgle Daemons, and I am sure I am missing other poison type attacks, and I am sure we will see more around in the future as well.
