A 17+ level rogue is doing something like 9d6+Dex at this point, extra +1d6 with TWFing or crossbow expert, with minimal chance to miss landing sneak attack
Also, you can't stack the same turret.
And a lvl 17 Artilarist is dealing 4d10+5 Firebolt damage with a +13 to hit, plus either 4d8 Balista with the ability to push, or up to 12d8 fire spread out amongst up to 12 enemies, or healing the party 2d8+10 health every round, or any combination of those, all as a bonus action that the Rogue is using for either his second attack or to disengage and run away. But at the same time they also are granting half cover to everyone nearby, which is not specified to only work on ranged attacks so it effectively grants a +2 to everyone’s AC and Dex saves, and have four Infused Magical items besides their Enhanced Wand that they are either using themselves, or giving to the rest of the party, all giving passive bonuses. Plus being a half caster. Meanwhile the Rogue has to make sure they even HAVE a Sneak Attack option, which they don’t always.
No, you can stack the same Turret. It says you CAN have different Turrets, not that you have to.
IAlchemists can add their Int roll to either healing or Acid/Poison damage, so Acid Splash and Poison Spray become viable, and therefore makes him more SAD if those are the main form of combat.
Yeah, a repeating Heavy Crossbow is awesome, but really only the Battle Smith gets the most out of it due to using Int instead of Dex. If any of the others try to use it instead of their own skills, they have to up both Int and Dex to accomplish the same thing.
The flamethrower is a 15 ft cube, which can catch 6 people, per turret. So it’s 1d8x12, potentially, or 2d8x6 if you hit six people twice.
I'd never count on getting 6 people in a blast every round.
You also don’t count on hitting your opponent every round, we’re looking at max potential. If you only have 1 enemy, then the flamethrower isn’t the best use of your turret. But if you have 2, the damage equals the ballista, just spread out, and if there are more than 2, then you are doing more damage than the ballista can, up to 6 enemies per ballista.
Assuming the flamethrower will hit 6 targets, is like assuming that both (or even 1) of the ballistas will score critical hits. It's not likely to happen.