They feel pretty fragile.
I don't know how you're playing the game but, if it is true that my player get sometime heroic gesture, most of the time they work as well as they can to avoid fire.
Yes SM are pretty capable to endure a lot of pain but they are only 4 and the ennemy too many.
What was your stigmartus has as weapon and or support, witch horde magnituide did they get.
Sure if 1000 stigmartus is a 40 horde in your game then my player could also kill them.
If you are playing opposition as me a regiment of stigmartus would have been between 4 or 5 30 magnitude horde and will have 4 to 5 chimera by horde, plus 4 or 5 15 magnitude horde with heavy weapons and like 12 to 15 elites and 1master. And maybe some leman russ to escort and provide support.
Then after fighting such a battle like berserker my player would be dead. But after playing like "soldier" using cover, hills and valley, flank attack and fast retreat, luring some part of the regiment there and an other part there, attacking small part before withdrawal and so on they will defeat their ennemy.
Have some heroic action is fine but don't play ennemy like stupid bad ass not knowing an iota of tactic and war.