It’s not just the stereo effect that can be hard to predict with omnis but their potential for creating a 3D “holographic” sort of image of an instrument being played. Dipoles by comparison are relatively 2D but IME still more convincing than any box speaker I’ve heard.
This illusion, for that is what it is, depends on the relationship of the omni speaker to the room which affects the amount of delay, diffuseness, and reduction in volume of multiple reflections reaching the listening position after the direct sound. Get the positioning wrong and a pleasant but slightly diffuse, sound could result. Get it right and the image will snap into focus with a tangible presence. To understand what an omni can achieve requires knowledge of, and positioning within, the room, something that no klippel or anechoic measurement can provide. The illusion also depends, more so then other types of speaker, on the hearing of the listener. If there were ever speakers that needed auditioning in the room in which they are going to be used they are omnis and to a lesser extent dipoles.
Reactions to hearing my speakers at my house have ranged from “Good god it sounds as if you have an actual piano in here” through “ah, that sounds like music being played” to “Well I can see that it makes a big pleasant sound”. People’s hearing and perceptions vary which is why the idea that one can get all one needs to know from measurements is, imho, essentially false and very short sighted, or should that be short eared.