Actually Balloons were also dangerous for friendly aircraft, in fact in England many more friendlies crashed in them then enemies, if to trust
http://www.bbrclub.org/Barrage%20Balloo ... 20WWII.htm
According to that site, in UK 310 friendly planes hit the cables compared to 54 hostile and 10 unknowns. That caused 91 friendly crashes and 38 friendly forced landings, while 172 friendlies made it back home to land normally with light damage. Only 25 hostile planes crashed and 1 forced landing, with the rest able to fly afterwards.
However it was a real scare for german pilots to encounter balloons, as cables were not visible. On the other hand, when german planes were not able to find their main target, they looked for balloons, and discharged there - there ought to be something important if they protect it by balloons.