In the May 2019 Unearthed Arcana update, two of the four Artificer specialist options can summon constructs as companions: the Alchemist summons an "alchemical homunculus", and the Battle Smith summons an "iron defender".
Unlike the Artillerist's turret, which is a magical object, these companions are considered creatures, and there is no text that says specifically what happens when they are reduced to zero hit points. If they die, reviving them requires expending a spell slot, so, I want to know how easy it is for them to get killed in battle.
When one of these constructs is reduced to zero hit points, does it die immediately, or does it start rolling death saving throws like a player character would?