In the visual world a holograph is typically something that has 3 dimensionality in appearance, but is also somewhat ethereal, lacking solidness. You can partially see through it. It’s obviously has no substance. My idea of something sounding too holographic would be sound that lacks fullness, solidness. What comes to mind for me is some piezo compression drivers I tried years ago. They sounded magical in a way that was hard to describe. Ethereal and holographic were words that came to my mind. These were on large horns and went all the way down to 800 Hz. When I ran a sweep and measured I saw that they had an average flat response from 800 Hz to 20 kHz but there were lots of narrow but very deep notches in the response if you turned off the smoothing. Those sounded too holographic to me. With some standard compression drivers the response without smoothing had no notches, and sounded very solid.
Most people don’t use drivers with deep notches in their response. But we do end up with notched response at our ears due to crosstalk between left and right channels, and early reflections. More notches that aren’t so deep are going to produce a fuller sound than a few very deep notches. I think I can always hear the lack of solidness with a 2 channel phantom center. The best way to deal with it is to break up the strongest early reflections into multiple smaller reflections, and delay them if possible.
Breaking up the center of the wall between the speakers with a center treatment of some kind that causes diffraction will bounce some sound straight back from the middle of the room to the listener, helping to lessen the effect and make a more solid center image.
If you listen to a 2 channel system in an anechoic environment, sounds that are panned to one speaker or the other will have much more solidity than sounds panned to the center. When done effectively, inter-aural crosstalk reduction can very noticeably solidify the center image, giving it near equal solidity to left and right panned images.
A properly set up multi-channel system playing appropriately mixed content should sound much less holographic (my definition) and more solid and clear across the entire sound stage. The inter-aural crosstalk is the worst offender because it happens nearly instantly across the head, about .5ms. Later reflections that come from other directions are much easier for our brain to tease apart.