First, thanks for all the answers. This is our first foray into Artificer, so the group is wonderiing why it's so good. I was curious where it balances. As far as competing with the wizard in our party, our wizard has 2 levels of rogue so, at fourth level, the 4th level is out-doing the wizard, obviously. Looks like the wizard will eventually out-shine the artificer at in spellcasting at higher levels.
The thing with spamming temp HP is that it's hard to drop anyone down to 0.
the barbarian is getting 7hp(on average) per round. It means he has to take 14 damage before it even eats into normal HP, and then, the next round, those HP are replenished. To boot, it's topping people up all around. It just makes everyone near the turret that much more tough to drop. So, sitting it near the barbarian is pretty potent and the barbarian can protect it.
The player hasn't used it this way because a) He thinks it's boring and; b) he didn't want to munchkin it up and he thought it would be too powerful.
The flamethrower is great. Sure, it has to get into close range but an AoE 1st level spell as a bonus action has done lots to help us. Especially considering we don't have a full spellcaster yet.
That 15 HPs is just a few HP worse than the rogue and it has a higher AC. So, it's as tough as some of the squishier PCs. The fact that an enemy has to use an action (or two or three) to drop it is just a boost to the party's survivability.
For the record, we were going to put the balista turret on my Mastiff mount (who, through RP, is intelligent - essentially, an NPC). We thought that would be cool...as long as the dog consents to being mobile artillery.