I know the difference, to have a player collision rotate on all axis is a quaternion and thats what the implication of gravity of a vector is not to mention the camera attached to a 6dof body if youre doing something other than a fixed camera. There are instances where you wont need to rotate collision bodies like spherical collision but its very specific and means the best solution to this problem would be a gravity vector implementation that sat along side the current one.
As a fixed gravity vector obviously you can move planar and it doesnt cause these types of problems but as gravityZ is adjustable having a similar implementation means a player could walk around a sphere which does indeed mean quaternion rotation! But feel free to share your math if Im wrong 
(Oh a technical note Gravity is not actually a Vector!)
Ive tried this and it still requires heaps of setup time, it doesnt stop the bugs in the process either like trying to reimport a StaticMesh with no material but youve assigned one in the editor. I do think they have added a commandline importer for batch importing Ive yet to try as well but the main issue here is that editing a StaticMesh/SkeletalMesh etc after import requires going into each asset with no ability to batch edit. The import options could be improved somewhat too so its not a report view but more of a table of inline entries 