Being transparent is good because users were able to see the status of the casino incase there's a gambler who made a decent profit in a certain game. But doing so is risky too because they might become a target of hacking.

Anyway, the bankroll is not solely the reason for the casino to become trusted. Because it's also about their reputation that they established. Plenty of casinos are not transparent but still operating for years already and tagged as one of the trusted casino sites.