Some peoples' skin is very sensitive to it....my skin is fairly sensitive to some chemicals, but fortunately bleach is not really one of them. The only real drawback I hate about using it is breathing in the fumes and getting the smell of it off my hands afterwards.Bark can rot in the tank as well much more easily then the hardwood in the middle.
I know bleach is chlorine based and that it gases out over time. But having been burned by it myself as a human I just cant help that having it around my tanks or decor makes me ancy.
Also, very important - NEVER USE BLEACH WITHOUT GOOD VENTILATION. I did it once when I was gutting my Anacondas cage a few years back and I almost passed out cuz I forgot how long I was bent over the side of the tank trapped in there scrubbing the inside of the tank and all of a sudden it just hit me and I was like "Holy **** I need to go outside"