Walnut isn't that weak of a link you can seal it up and a good full lenght glass bed job will keep it from warping enough to keep it from POI shifts. Wood stocks have been used on rifles for longer than synthetic and the warping issue isn't a new one, people have still hunted and shot in competitions with success with wood stocked rifles for years.
Truly the most stable stock is a synthetic that is laid up and not injection molded, followed by a lamianted wood stock, and then a traditional wood stock. A laid up stock synthetic stock will still expand and contract a little with extreme temperature changes, but because it is basically laminated as well and the grains aren't all running the same direction it isn't going to be able to move as much. However we are talking hunting rifles and a properly sealed and bedded wood stock shouldn't change your POI by more than 1/2" in the most extreme temperature variations.
Just look around at everyone that hunts, I'd bet the vast majority of them still hunt with a wood stock, and do so in extreme temperature and humidity shifts all across the world. I've hunted upstate NY, to Sout TX, all of Colorado, and the coastal rain forrests of Southeast AK. I can tell you all rifles and pistols will rust regardless of being blued or SS and there isn't one of thoese places I wouldn't take a wood stocked rifle. You should check your zero every time you travel to hunt just to be sure something didn't change during travel.
The best way to have a rifle not rust in a humid wet climate is to have the metal coated. Black T, Cerakote, Gunkote, and a host of others all prevent rust better than bluing or plain SS. Matte/Satin finish SS rifles surface rust just as fast if not faster than blued rifles in my experience. The reason for this from what I've been able to find out is because manufacturers don't change out their sandblasting media when they give them the matte finish. The media has small particles of chrome moly in it from blasting CM barrels and actions and it gets imbedded in the pores of the SS rifle. Thus causing your SS rifle to get surface rust.
I've experienced this twice in Alaska once with my buddies SS Contender pistol and my SS Encore rifle. First time I went I carried a .30-06 ADL and it showed no signs of surface rust after a week and my buddies Contender pisto almost rusted shut and we oiled our fireams daily, he just forgot the hinge pin and he kept his pistol in a holster. This last time another friends wood and blued A-Bolt fared much better than my SS Encore for five days of hunting in AK. Both times neither of my rifles showed surface rust until after I got home to CO, but the Encore had more area affected by the surface rust.
I guess my point is that you have to take care of it regardless if the rifle is SS Synthetic, or SS Wood. However wood stocks may take a little more maintenance but that doesn't mean that they can't be hunted with in all enviroments. If SS and wood is what you want then get it because while it take a little more maintenance you can still hunt successfully with it.