I think this is the key takeaway from this article:

Neil Levy, deputy director of the Oxford Centre for Neuroethics, said that compared with prescription drugs, electrical brain stimulation could actually be a safer way to boost the performance of those in the armed forces. “I have more serious worries about the extent to which participants can give informed consent, and whether they can opt out once it is approved for use,” he said. “Even for those jobs where attention is absolutely critical, you want to be very careful about making it compulsory, or there being a strong social pressure to use it, before we are really sure about its long-term safety.”