In the Unanathed Arcana update of May 2019, two of Artificer's four options may invoke constructions as companions: the alchemist invokes an "alchemical homunculus" and the Battle of Smith invokes a "ferrous defender".
Unlike the gunner's turret, which is a magic object, these companions are considered creatures and there is no text that specifically says what happens when they are zeroed out. If they die, to revive them, you have to spend some time for spells. I would therefore like to know how easy it is for them to get killed in combat.
If any of these constructs are reduced to zero health, does it die immediately or does it start throwing save throws as a player character?