Not knowing much (yet) about machining, I would have thought on a lathe, the flywheel would turn, the machining bit would stay still, and any run-out or unevenness would be easily seen as the flywheel rotated on the lathe.
But, the finished product is only as good as the way the machine operator fixes the flywheel to the lathe. But once again, any run-out would be noticed straight away as the flywheel rotates.
I have no idea how a surface grinder works, or what it looks like, but once again the finished product would depend on the flywheel being perfectly flat when the machining starts.
I pick up my lathe on Friday. and then start to learn how to machine inlet manifold tubes that taper from 40mm to 32mm from the 40 DCOE Weber to the head, in the length of the manifold from carb to valve seat.
Alex in Norfolk (a machine operator with L plates).
P.S. using the milling machine has taken me about 20 hours to work out.