Yes, if the driver is hardly moving its a lot harder for it to distort. Diaphragms can break up despite that, causing distortion. That's the one thing that can be obnoxious on a horn midrange. In the old days this was common (the aluminum diaphragm of the Altec horns has this problem at higher levels). My horns use a beryllium diaphragm which has its first break up at about 35KHz, so its pretty smooth.
But in a home situation the side walls are a constant menace to smooth sound. Early reflections like you get with side walls are interpreted by the ear as harshness. Horns have a controlled directivity so can be used to minimize side wall reflections, resulting in smoother sound.
And corner horns with open backs use the front and side walls as extensions, I believe increasing sensitivity and also their ability to go lower in the bass region. My speakers' directionality basically seems to minimizes the side wall reflections. They point to the opposite rear corners of my near square room and then the reflections get broken up by those far away and irregular corners. The speakers behave very differently than my former speakers in that room.