Soundstage is 3D, or you can call it holographic imaging.
Left to right stereo separation is the first dimension. IEMs can do this fine, although everything is inside the ehad.
Height is the second dimension. IME I never experienced much soundstage height with IEMs.
Depth is the third dimentsion. It is pretty hard for any headphone to project any soundstage depth, not to mention IEMs.
When you have 3D soundstage, not only can you locate the instrument location, you also feel the air around it, as well as room ambience. Even the best IEMs like UE-10 fails pretty short of this goal.