I think of Vincent Walsh as the most skeptical of the tDCS researchers. You can get a clear understanding of his doubts and concerns in this video from the Davis Summit on tDCS from 2013. https://www.youtube.com/watch?v=9fz7r8VDV4o
However, Vincent Walsh, a cognitive neuroscientist at University College London, is less convinced. “This is an important paper,” he says, especially because it casts doubt on the aspect of this research that until now had been assumed to be the most robust – the physiology.
“In terms of cognition, which is the other aspect that people make claims about, tDCS is massively hyped. The danger is that people have been promised better memories, better reading, better maths, increased intelligence… you name it. The effects are small, short lasting, and no substantial claims have been replicated across laboratories. This paper is hopefully the beginning of a counterweight to all the bullshit.”