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.
There was an electric fence at the end of my street as a kid. My friends and I played with it quite a lot :)
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.
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.
You can kill yourself with a 9 volt battery. There's no reason not to be skeptical. And if you aren't careful with tdcs, you can burn yourself pretty badly.
