I would expect that bringing in groups of people weakens the influence of idiosyncratic biases (as well as idiosyncratic rationality) and strengthens the influence of shared/conventional biases.

I wouldn't expect it to reduce subjective bias per se, though.

Note also that appeals courts are intended to counter the most egregious idiosyncracies... though they aren't the same thing as having a council of judges making the decision in the first place.