Plastic combs are molded, or "cast" so to speak, and in low-cost mass-produced harmonicas are not further refined, so many fronts are not true and level. They need to be trued as described below. Even Suzuki's Sirius combs are cast, and I don't see any signs of work on the front after molding. (I assume they use better molds for a more expensive harmonica.)
As a matter of fact, unless the player slobbers into the harp, wood is just about as stable, if not more so, as plastic. Hohner uses pear wood for its wooden combs - a hardwood that doesn't swell or shrink much, and is probably a better choice than most plastics which swell and shrink with temperature change. I have and play a 260 that's at least 25 years old and the pearwood comb is still perfect, its varnish pristine. I recently sanded the front on a small piece of plate glass (5" X 8") that the local glass man made for me, which I use to adjust woodwind MPs and reeds, using 600 wetordry held flat with my hand. (Plate glass is precisely flat.)
An interesting fact about wood swelling is that it does not swell or shrink end-to-end but only across the grain. That's why carpenters' rules are made from hardwood and not plastic - they maintain their accuracy of length and plastics do not.
Gnarly always says that if you want to play harmonica you have to learn to tune them and change valves and stuff. Well, leveling the front of a comb is a lot easier than that stuff.