Yes to your first question, or at least that's where most literature leans. In brain stimulation research the terms online and offline refer to whether stim is delivered concurrently with a task (online) or before/after a task (offline). Generally greater benefits are seen in studies that repeatedly pair brain stim concurrently with a task over several sessions, but of course many methodological factors (intensity, duration, electrode location) and individual factors (brain morphology, neurochemical concentration levels) can influence the outcome.

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.

I would (slightly) disagree with that.

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.