I am assuming that, by Horizontal, you would be laying your keg on its side and would have some legs to keep it from rolling. The big disadvantage that I see is that your grain bed would not be a uniform depth. This might not be a problem if you batch sparge, but it would be a huge problem if you fly sparge, because the curvature of the mash tun would cause the sparge water to channel through the grain bed, which is what you want to avoid.
If I were to use a keg for a mash tun, I would use it in a vertical position only.
This is my two-cents worth. Mark