>Is it only the sound stage or are the other differences?

The difference is the sensation of the sound around your ears (especially the bass). You may think that this is a minor thing, but it's not. Yes, this affect soundstage sensation.

The only other difference lies in the frequency response of the two devices. In-ear monitors (IEMs) have better isolation and do not interact with the outer ear, allowing them to deliver a good volume with "less effort". This makes it much easier to create IEMs with excellent sound quality and considerably low harmonic distortion, allowing for easy equalization.

Some IEMs with multiple BAs have slightly higher harmonic distortion. This type of driver is mostly used in high-end IEMs, while headphones (IEMs) with dynamic drivers have considerably lower harmonic distortion, reaching values as low as ≤0.1%.