Uhm no Arty (and kira since they are pretty close) isn’t terrible he has a higher overall skillcap than skyhammer that’s all.
Skyhammer:
Is a jack of all trades his weapons are ok to good on most distances and being full auto means they are ok with some missed shots. His airstrike deals a huge amount of damage in open spaces and is fire and forget. On top it deals a meager amount of damage in closed space. All in all his playstyle is very forgiving and he is the closest to every other soldier character in every other FPS making him easy to pick up. which is the reason you get him for free and not Arty.
Arty:
His weapons have a well defined effective range forcing you to keep your distance.
Or taking your chances in a close encounter. Also they relay on utilising ADS and actually scoring Headshots (Burst rifles can oneshot Proxy, Aura, Kira Dreiss two shots those and Vasilli).
His artillery does takes some skill aswell. As others have stated targeting can be quite the diva. And to be most effective with it you have to know the map. But you can hit quite alot of corners without being seen (look for walls behind your target).
And the biggest secret to being effective with the artillery is to only place one marker at a time (except on the EV).
One artillery will kill all deployables and most mercs. Everyone you didn’t get with the first wont be around for the second so setting 2 at the same location is plain wrong.
Placing 1 is just as effective and ensures that you always have one artillery up.
2 Artilleries on the same location is reserved for the EV. If they land perfectly you will take it out even if they don’t they will have so low HP that a fragger/fletcher/Nader nade or simple gunfire will take care of the rest. And in the worst case you are 20 sek. away from your next Artillery.
Guess who has to wait over a minute for his next try if he botches. And let me tell you this in a minute the EV can cover quite some ground.
Underground isn’t so bad either aside the fact that the whole merc system revolves around taking the best mercs for the job (i.e. feel free to not play arty or kira on underground).
But they can still positivly affect the map:
denying the enemy the roof over the first objective, slowing them down in the elevator section (shoot the roof first) and making planting/defusing a nightmare.
Is he perfect nope, could he use a buff sure who doesn’t, but he is way better than most people make him out to be imho.