Brakes opening on tow is not a major safety issue. What can become a major safety issue is the tow being continued with brakes open because the pilot was simply unaware they were open. That cannot happen if the airbrake handle is guarded (hand on, or immediately behind, the handle).
Use of airbrake was my preferred method of managing slack rope on aerotow in the ASW19b or ASW28. There is no trim change or other upset when the brakes are opened only an increase in drag. Since I use brakes for takeoff (improved low speed roll control), and for slack rope management, my left hand is usually on the brake handle for the entire tow. Those who know these gliders will know that a hand on the closed airbrake handle is also very close to the release.
I've spent a bit of time at the front of the rope too. Citabria, Scout, Pawnee, and 180 Super Cub in USA and Citabria in UK. I never had to dump anyone or wave anyone off but a Standard Cirrus pilot who pulled up aggressively and turned away without releasing the rope got my attention once! The Cirrus back released about the time I was starting to pitch down. We had a discussion when he landed. He said he had pulled the release and was not aware that the rope had no released. The discussion continued!