I find it so funny though...with all Bruce's "preparations" HE is often the reason why the crap hits the fan from his protocols Talia stole to Brother Eye. And him not sharing was usually part of the problem. Again I ask who keeps Batman on his toes? Why do writers assume Batman has to be the moral authority here? It's insulting to the others imo. Citing Clark and Diana are powerful and deserve that...this is the DCU...it is a universe that has many metas and Gods running around. That makes no sense to me. He should have contingencies for his crazy foes and even Lex and other metas then. It's writers who often have Bruce behave like the arrogant dick and not even admit how wrong he is/was to not share his plans. The idea Clark docilely accepts keeping the whole thing secret does not make it any better. In last canon Clark was angry Bruce had stuff behind the others backs. I don't like how DC rarely has Clark not standing up to Bruce especially when it is something he would not do or just accepting Bruce talking to him as if he is a child. The constant finger wagging would piss me off.
I think while Bruce will not abuse Clark's trust ...least I hope not...but you never know with his mental state...chances are just giving it over, there is a risk that someone else could get hold of the key. But Clark is always at risk of one day never coming back after a battle so it makes sense he gives someone a key. So really as far as it backfiring...it wouldn't be anyone's fault unlike the contingency plans etc. Bruce and Diana need to simply be as careful and not share that they have a key with anyone else. I mean it's not like if Clark's FOS was never breached before and it was not their fault. Look at Zod breaking out his cell and messing around for example. Threats can come from other places too. As others have said apart from a key, other security measures could also be put in place as well.