ill have a go at explaining what i think the issues may be...
stainless is a bit of a bast**d to machine and needs a good solid heavy machine (slow and cumbersome as you would imagine)

wood needs speed (high spinlde speeds and feed) and requires much less cutting force so a less sturdy and lighter machine

to some degree you can take very light cuts on a light machine to cut harder materials however i dont think a typical cnc router will manage stainless because to have your cuts light enough not to cause resonance problems the tool probably wouldnt be cutting at all and would be just rubbing the tool to a very early death
(if it was cutting you would be there all day making a small part)

inversly, cutting wood on a slow cumbersome mill would not feed into the wood fast enough to get a good chipload and would rub/heat the tool... again, an early death for your tool

soooooo.... its either a very heavy built router or a mill with massive motors

hope this gives you an idea of the problem... its not impossible to solve the one size fits all but it sails pretty close to the "two machines is cheaper" solution