The main hazard on a banner pick up is getting the tow rope caught around one of the wheels, which means you can't release it, which can cause a multitude of problems. The other hazard is catching the banner on an obstruction during the climb out, my recollection was the performance data was very inaccurate, sometimes you were lucky if you could get 150ft/min climb. It's supprising that the CAA didn't ban it years ago, or at lease review the safety aspects.