GUV's are not placed in any order that a user can edit directly. The polygons are taken by vertex order I believe and placed on the UV tile. So you could have a polygon from the foot next to a polygon from the chest. It's nearly impossible to edit in a traditional paint program. And yes, PTEX equally so, which is why most apps convert the PTEX data to standard UV information when sharing files between programs. This will probably continue until more programs adopt PTEX (assuming more programs will).