These things will engrave metal including stainless you just have to go at it slowly. From my research there is a huge variety of CNC3018-Pro machines out there with differing control boards being the difference.
Some have later driver chips on with a finer resolution, some appear tohave the row of pins A1 etc on the bottom disabled which is not helpful if you want to use a Z axis mapping function.
For that reason I shall probably be buying the Sainsmart one which although a little more expensive has the latest iterations of components and supposedly reasonable customer support.