Of course there's potential and loopholes but the basic observation (dubbed 'Algernon's law' by some; I've written an entire essay on the topic: http://www.gwern.net/Drug%20heuristics ) seems sound for tDCS: it's such a trivial intervention that it shouldn't work, which suggests there must be some catch.
The catch may well be trivial from our modern perspective, in the same way that staying up late or using stimulants and so burning some more calories is a trivial catch from the modern perspective ("'oh no', said the fat American man, 'you mean X might cause me to burn some more calories? how awful'"), but until the catch has been identified, tDCS has not been fully understood.