I suspect that a crux of the issue about the relative importance of growth vs. epistemic virtue is whether you expect most of the value of the EA community comes from novel insights and research that it does, or through moving money to the things that are already known about.
In the early days of EA I think that GiveWell’s quality was a major factor in getting people to donate, but I think that the EA movement is large enough now that growth isn’t necessarily related to rigor—the largest charities (like Salvation Army or YMCA) don’t seem to be particularly epistemically rigorous at all. I’m not sure how closely the marginal EA is checking claims, and I think that EA is now mainstream enough that more people don’t experience strong social pressure to justify it.
I think that the wisdom of nature prior would say that we shouldn’t expect blasting a neurotransmitter pathway to be evolutionarily adaptive on average. If we know why something wouldn’t be adaptive, then it seems like it doesn’t apply. This prior would argue against claims like “X increases human capital”, but not claims like “X increases altruism”, since there’s a clear mechanism whereby being much more altruistic than normal is bad for inclusive genetic fitness.
I would worry about this more if the OP were referring to a specific intervention rather than a class of interventions. I think that the concern about being good on longterm and shortterm perspectives is reasonable, though there is a proposed mechanism (healing emotional blocks) that is related to both.
Normal drug discovery seems to be based off of coming up with hypotheses, then testing many chemicals to find statistically significant effects. In contrast, these trials are investigating chemicals that people are already taking for their effects. Running many trials then continuing the investigations that find significance is a good way to generate false positives, but that doesn’t seem to be the case here, and I would be surprised to find zero effect (as opposed to shorter or different effects) if it were investigated more thoroughly.
I also think that improving human capital is important, and am not convinced that this is a clear and unambiguous winner for that goal. I’m curious about what evidence would make you more optimistic about the possibility of large improvements to human capital.