These are small, battery powered devices. There is little that a "hacker" could do to hurt himself that he couldn't do better by sticking his tongue in a light socket.
So...on one hand, these are supposed to herald a bold new way of treating various disorders because its effects can be so powerful, but on the other hand, you couldn't possibly mess up and cause harm?
I don't think that kind of logic has ever been true, ever, about anything. Either it's inert or it's effective; inert has no upside or downside, while effective means it can be done incorrectly or abused, resulting in harm. Personally, I think the idea of zapping your own brain to alter your neurological functions is NOT a good idea from a "do-it-yourself" perspective. Some things are not suitable for unsupervised trial-and-error approaches.
As they say: if at first you don't succeed, skydiving is not for you.