Have you ever played with any electronics to the extent that you've shocked yourself? It's actually pretty innocuous for the most part. TDCS isn't using the kind of current that comes out of a wall outlet.
I was shocked with it dozens of times, we all were, and only once did it really hurt. I couldn't walk for a few hours. For some reason my left leg went numb and didn't feel right.
Boys will be boys.
I don't dispute anything of what you're saying in itself, but if you mean to imply that it's hard to go wrong with direct current stimulation to the brain because the currents are much smaller than what's needed to drive household appliances, I'm highly skeptical of that claim.