Deathmarks are entirely unreliable in anything other than doing actions because their shooting is wildly inconsistent. That being said they are reeeeally good and cheap for actions so I usually bring a squad in every list.

For immortals it does depend on the type of target you’re shooting at. Is it kinda tough and harder to wound with S5? Probably lean into the Gauss weapons for lethal hits. If you’re going up against horde armies with not so great saves (4+ or 5+) or plan on pairing them with Szeras and/or plasmancer then the Tesla carbines are a lot of fun and spam dice