OP here, the best times to visit are spring and fall, winters are relatively mild as well. Summers have it all: heavy humidity and stifling heat, skeeters, ticks, cicadas, and in Arkansas tons of chiggers. (Missouri isn't as bad with chiggers in my experience.) The vegetation is basically rainforest-tier in the summer, so if you go off-trail be prepared to bushwhack.
Beautiful region though, the land feels much "older" than the west, if that makes any sense. Very mysterious and brooding. The midwest is pretty lame when it comes to public land but the Ozarks are a jewel. If forests and rolling hills and hidden streams are your thing you'll love the area. (I guess technically it's a "mountain" region but 2000-something feet doesn't count as a mountain in my book.)