I've been using them in the campaign a bit now. They perform a bit better there thanks to technologies and general skills. Also gaining experience seems to help them considerably as they get some much needed accuracy and seem to fire a little bit faster. Plus, these are more realistic fights, whereas the testing early was just 1 unit vs 1 unit. With other stuff to distract the opponent they get a bit more of a chance to try and do something usefull.
They're still terrible in melee though, even against archers or low tier infantry. I've seen em lose against nightrunners. Against high elve archers they win, but lose nearly half their number if you just go straight for the melee (and get shot to bits at range). Hell, I just tried an oldblood with skinks vs an oldblood with salamanders and just throw em against eachother and even then the salamanders only barely won (my old blood nearly died, less than half my salamanders where alive and at max 1/4th of their health remaining). This despite salamanders being more than THREE TIMES as expensive as skinks.
As for the hellpit. It is weak to fire, so yeah, one of the few examples where the salamanders are of any use. Though if it gets into melee it'l destroy both salamanders and ancient ones. 2 units of salamanders quickly route and then kill it though, mostly cuz 3-4 volleys basicly kills the abomination with its weakness to fire. Which is roughly equivalent in cost to 1 abomination so there they trade fairly I guess.
What is notable though is that they appear to work significantly better when firing at smaller units (as in less guys in a unit), obviously massed fire would work here as well, basicly just try to have more projectiles than targets

. Their projectiles have knock back, so if multiple projectiles hit a target it can get stunlocked for a while. I had them fire at an elven lord on foot and he couldn't get to them anymore due to constantly being knocked over. They also appear to be doing more damage this way then when it's spread out over a 100 or so targets in a unit, maybe something weird with how damage is calculated when caught in an explosion in the game (best I can tell explosions do seem to damage based on how badly you get hit, and a single model unit like a lord or monster generally gets caught in more of the explosion then a big unit with 100 guys where only 1 guy gets properly hit and his friends only suffer collatoral damage)
This makes the ancient one even worse though as he only fires one shot at a time. He even performed worse against the abomination at range as he simply missed half his shots while the normal salamanders hit with say 80%. He did perform a little bit better in the melee though due to having armoured & terror as opposed to not being armoured and only having panic.