Simulacrum conflicts with Guardian Spirit in most conditions, where two effects trigger simultaneously.
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.