Some wireless keyboards don't bother with any kind of protection to the data stream between the keyboard and the wireless receiver. That's the most obvious instance of bad keyboards. However, these days most wireless keyboards do use some kind of encryption on the pairing between the keyboard and the receiver, so that is a bit of a moot point.
Even if the data stream itself is encrypted there's still a little bit of data leakage. Your keyboard isn't constantly sending data, it really only chirps when there's an actual keypress event. So if you look at the actual physical RF, you 'll notice patterns related to the user's typing. There is some research in trying to guess key presses based on typing cadence, although I'm not sure exactly how effective it really is.
I say all of this typing on a Logitech Unifi keyboard amd routinely use bluetooth keyboards. As others have mentioned it really depends on your threat profile, and in the case of wireless keyboards you probably aren't near the level where this paranoia is justified. Are you typing state secrets that a foreign government body really wants in a public place? Probably want to have a wired keyboard...or maybe just not type such things in such places. Are you typing out a comment on Hacker News in a private space? Probably have nothing to worry about with a wireless keyboard.
