i guess I shouldnt generalize, sorry for the standoffish comment. When I switched from over to under it was night and day for me so I assumed it would be the same for others.
In my experience, with my io7's, they dont sit perfectly on the bridge of my nose, and let air through near the nose unless my straps are super tight when they are around my helmet. Having them pressing into my face over extended period gets uncomfortable from the straps being so tight. Having the straps that tight has also lead to them fraying near where they connect to the frame of the goggles. So when I realized that some people wear them under and tried it myself, I was pleasantly surprised to fix both issues. Having the goggles under lets them sit at a more downward angle angle on the bridge of my nose, making a better seal, and the straps dont have to be tight at all. Much more comfortable for me. To each his own.