So it ticks all boxes for me as an amature enthusiast for a potential 'enhance my cognition' method. I wonder, where the catch?
Some of the authors' earlier work in rodents painted a really exciting picture of directly driving neuronal activity from outside the brain. However, that data came from an animal with a small, smooth brain and the stimulation was actually delivered onto the skull (rather than the skin). Thus, they could make electric fields of ~400 V/m in their animals, whereas fields of ~1 V/m are about all you can expect from non-invasive (i.e., scalp) electrical stimulation of an awake, healthy human (at least one that you want to keep healthy!).
We recently did some experiments with macaques receiving tTIS through the scalp, which are much more similar to humans. It could shift the timing of individual neurons' ongoing activity, but not directly force them to spike. It generally wasn't strong enough to impose new rhythms on the brain, just muck up existing ones. This isn't nothing--"mistimed" neural activity is involved in lots of diseases and ways to fix that might be very valuable--but it's very different fro the dominant story.
Here are our results, if you want to read more: https://www.biorxiv.org/content/10.1101/2023.09.25.559340v1
With your second question; brain stimulation is reasoned to be less addictive than those substances, and of course less selective in its effect. Furthermore the studies that report positive benefits can show these benefits lasting almost 24h following a single stimulation session....however, having done my doctorate on this technology I can tell you it is definitely no panacea, and lately many critiques of this type of intervention have been published.
The hope is often to enhance whatever's already going on in the brain--and this is a sensible goal because there's often less neural synchrony in some diseases (e.g, schizophrenia) or situations (e.g., memory lapses).
However, the brain has its own internal dynamics--and they're complicated! You can 't just blithely write a new thing on top of them and expect it to work. In our experiments, doing so often backfires: stimulating at 5 Hz often makes makes cells already firing at 5 Hz act less rhythmically, not more. The reason is that the brain's oscillations drift in frequency and abruptly reset, so they "compete" with the imposed oscillation. This is a pretty well-studied phenomeon in other contexts (e.g., sloshing in rocket fuel tanks or nearby pipes cancelling each other in organs).
TI is exciting because you could potentially sense what's going on and adjust the stimulation accordingly, but we've got a lot of work to go.