It depends completely on the plant, the processing (if any), and other factors. I know nothing about lemongrass but St Johns Wort for example is GABAergic, acts on serotonin and dopamine, and under controlled trials shows clinical significance.
Placebo is one of those factors and it can be a MASSIVE factor. Conversely, nocebo, the opposite of placebo shows that if patients do not believe something will work, even though it’s shown highly significant clinical success, chances of it working on them are far less.