All the commanders at Shiloh were equally exposed to risk, Johnston just had the misfortune of being hit. I acknowledge that it's all a matter of opinion though...
I agree with Wiley Sword's opinion: "Considering the fatal result, Sidney Johnston's exposure on the front line has been frequently interpreted as grossly reckless, and improper conduct for a commanding general. Yet, aside from considerations of misguided heroism, Johnston's presence along the battle line involved maintaining tactical control of his main offensive thrust. Thus, both his bravery and commitment to win should be apparent to all. Due to the tactical nightmare of mixed commands and random unit coordination, it was imperative that someone with high command authority be present to organize a cohesive attack. Like Johnston, the other senior commanders, Grant, Sherman, and later Beauregard, personally exposed themselves along the front line to obtain information, rally troops, and direct the fighting. All had close calls and were occasionally fired at. In fact, Johnston's limited exposure, and the random, chance nature of his fatal wound seems to have involved more ill-luck than a reckless abuse of command responsibility."
p447, Shiloh: Bloody April.