I hope I'm reading this wrong, but this paper's conclusions seem to make claims unsupported by a majority of the component papers (and the choice of component studies seems arbitrary in general, given that no two have the exact same strains under test).

10 studies spanning a mishmash of probiotics (and not even the same strains), 5 of which conclude no effect, and 1 says adverse effects on anxiety. Only two claimed significant benefit of any probiotic blend vs. placebo.

Only 3 had populations > 60.