It’s safe in the sense that the currents are weak enough not to cause thermal damage to the brain (though it can cause skin redness) or directly evoke seizure-like activity.

The whole point is to change brain activity, and obviously one hopes it does so in a “good” way, but ensuring that this reliably happens is a much thornier problem. One of the earlier products, which was supposed to make you better at video games, actually made people slightly worse!