Lived in MO for 40 years and roamed the woods like a feral cat growing up. Only recall seeing a couple copperheads in the summer and cotton mouths while on float trips near the Arkansas Missouri border. I used to hunt during archery and rifle season and never saw a poisonous snake. I caught this big black snake while doing some work on my moms barn a couple years ago. Black snakes are not poisonous and do a good job of keeping mice and rats out of your barns but this one was getting a little aggressive and scared my mom pretty good a few times. I relocated him to different areas of the farm 3 days in a row and he kept coming back. If your hunt is before the first frost (usually around Halloween) the snakes and bugs might still be active. After the first frost you'll only have to worry about the mice, raccoons, and meth heads.
My experience during archery season in the Ozarks suggests snakes are not a big deal, but ticks and chiggers are, especially early. I would avoid floor less for that reason alone.
I’ve only come across one copperhead in the 13 yrs I’ve been hunting here... seed ticks/ticks seem to be the worst predator we face in the early season.
Bugs will be the main challenge. Will need to treat clothes and sleep system with permethrin.
I have done floorless in Missouri fall and spring seasons without issue as so many backpackers. A Borah bug bivy is an alternative to pesticides that will help.
I would go with a floor 100%. Morning dew in Missouri can seem like you got an inch of rain during the night. It's not like Colorado or western states where it's bone dry when you wake up. With that being said, it isn't a bad idea to make sure your pants and shoes are waterproof as well. If you are walking through thick areas and your not waterproofed you're gonna be waterlogged. What part of Missouri will you be going to? I'd be glad to give you some tips if it's central mo. Send me a dm.