It's not a hot air balloon, though, they can only really control altitude by dropping ballast, and that only works once. They follow the Jetstream, too, which describes a variable but very definitive direction. Individual layers within the Jetstream follow a similar path, like a turbulent river. If the jetstream overflies an airbase or something, cool, if not they are out of luck.
Variations in direction of the Jetstream are difficult to predict, which, given its role in driving weather systems, is why forecasts at mid latitudes (like us) are unreliable.
In general it's just a crummy system which only has merit as an unpredictable tactic.