Skyhammer and Arty occupy a very similar gamespace, but their abilities have very different effects on tempo.
Skyhammer can cause massive momentum swings, such as ensuring the attackers get enough space to get things done (i.e. Chapel stage 3, clearing the bus or spawnstriking) or stopping the attackers in their tracks (i.e. any EV).
Arty gets many more strikes that cause smaller momentum changes. Rather than winning a fight by taking out the entire enemy team, he can give his team the edge by taking out one (i.e. that pesky Aura on her healy box) or forcing the enemy to move (i.e. the one crouching behind the EV).
The choice between them in pubs is largely preference. I prefer my Dreiss and having the utility of taking out secondary objectives, so I mostly play Arty.
In coordinated environments, the choice has more factors. You’ll want at least one of them on an EV map, but having one of each isn’t a bad thing. Your team might only have space for one, and having the Stark to dance at range is a really nice proposition. Arty also has spare strikes when the EV is already down, which only makes him more versatile.
No fire support is bad or disposable. Part of the reason Skyhammer has a bad reputation is all of the Babyhammers that don’t know game mechanics. I would argue there is a second part to this, one I only thought of while writing this:
A good fire support goes unnoticed.
It’s very apparent when someone isn’t pulling their weight in a team-based game, particularly when murdering cannot occur due to a lack of ammunition. It may be very apparent that one player is significantly better than their opponents. Someone in the middle of the pack is just another merc that was (hopefully) satisfactory in their job.