Health amount recovered in spirit form, except for that gained from a very brief period before duration ends, will be neglected when Simulacrum expires. A Necromancer may find exploit here to achieve "immortality". One of the easiest approach is to use gears with "Heal on hit" property (e.g. Fade-Touched Snoufleur Skin) and cast spells of multiple hits (notably Energy Barrage and Chain Lightning) during that brief time window, hence health amount recovered can revive Necromancer afterwards.
Note that the mechanics will not work when soloing.
Triggers simultaneously with Guardian Spirit (if unlocked), resulting in both effects being consumed to do essentially the same thing. The only thing Guardian Spirit does (if Simulacrum is also unlocked) is that it adds a full barrier (which is basically useless). An unofficial fix for pcis available here.
Behind the scenes, Simulacrum basically makes you immune (even before you get killed and trigger the ability). Because of this, characters who have learned this passive, and while it is not on cooldown, will not receive damage higher than their current health. This means that if, for example, Dorian (who has learned Simulacrum) is down to 1 health, enemies will deal only 1 damage to him (even if he has Barrier up). An unofficial fix for pcis available here.