In-wall speakers have a different bass loading and different directivity issues, with (obviously) fewer issues with backwards radiation, cabinet resonances etc. They are, if designed properly, IMO inherently good speakers design-wise. They aren't widely used though for the obvious reason - they need to be installed ideally at the same time the room is built - natural enough for studios, not so much domestic living rooms.
So there tend to be very few good in-wall or on-wall speaker solutions for home use. There are some though, and IMO are worth looking at unless you really do have plenty of space
For home cinema use, I'm aiming specifically for Monitor Audio Shadows in the short term and, in the long run, real in-wall active speakers - s/h Meridian A350s if I can find them or mabye DSP 420s. Domestically perfect - NO boxes, stands or cables. Properly installed and well integrated into the decor, these are the way to go IMO
And yes, I suspect that 'invisible' speakers sound different, because we aren't mentally pinning the sound on them simply because we can see them. We are much more reliant on the sound itself to inform the soundstage, only without the visual 'anchors' leading us to 'expect' where to find centre-stage. I suspect then that centre-stage even with a good hifi is NOT as 'central' as people might think.
Another point is the wall itself - it can be a major source of reflected sound... I'm not sure how that relates to this topic, but I think it's a factor in the sound relating to speaker placement.
Interesting video with Bob Stuart on their in-wall speakers.