In most weather conditions you can drive a OneWheel.
In icy and snowy weather it is obviously less appropriate to use your OneWheel, unless the trails are cleared of snow and salted. In this case you should clean your OneWheel after each use because salt has a corrosive effect.
In cold temperatures, you should consider a slightly lower autonomy of the battery.
If weather conditions permit to ride a bicycle, you can be sure it's safe to use a unicycle.