For one thing, in the case of tDCS one is actively trying to penetrate the insulating protection of the skin. For another, nervous systems change in response to what happens to them, particularly when what happens is more extreme than the everyday state of affairs.

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.