I think it's caught me out in the past, but I still see work-plane warnings occasionally despite being careful not to touch the 2D/3D toggle, so I don't know if something else can cause them to appear.
To be honest I had no idea that Safe UCS mode exists - but from your description it's not an obvious control, and is right where you don't want to venture (in 3D Properties??). I suppose if you are trying to work only in 2D with default settings, you don't expect work-plane and co-ordinate oddities; but I have known objects to occupy weirdly displaced co-ordinates of their own.
I base all my drawings on the default World Co-ordinates; and certainly so when I tried to learn 3D drawing; working outwards from the primary centre- or datum- lines being the (x,y) axes, for the whole drawing. It means a lot negative numbers (even harder from (0,0,0)!) but is safer than trying to define an origin elsewhere. (Though this assumes the grid and rulers do appear - they seem not to, on some of the templates.)