There's legitimate concern over sideband channels if the two are in close proximity, but you can just change the channel of either to avoid interference. I've never had an issue and I've never seen anyone say "man, this Zigbee/WiFi interference is just insurmountable". It's not an issue that doesn't have a simple solution, in other words.
Keep in mind that Peanut plugs need a Zigbee channel below 20. That's just a requirement to get those plugs to work properly. Oh, and I think I recall reading that they are not good at repeating Zigbee signals.
