Having 9 years experience in Working with Anti-cheat programs, you quite honestly say nothing is safe nor 100%. well your correct, you can only carry out preventive security measures when you admin your servers and have constant updating from the Anti Cheat Program. Other back doors that also causes or encourages cheats and hacking is the private community made maps etc, which if not controlled and checked properly by the ACP do'nt help either!. Vac is a waste of time as this has been bypassed long ago as it do'nt detect private cheats or hacks!!.
At the moment in what I know an Arma 3 hack came out on the 25/03/2013 and has been used and viewed by at least 632 people and it is one you have pay, day light robbery he If I say so !!.
So I would look at Battleye being brought back and give sebastian more help and info so that more updates can be brought out asap, but this depends on what Sebastian says from Battleye !.