The answer is quite simple: unions cannot be right even if they are, empirically. The solution to this dissonance is to ignore them.
When a union rep brings up safety concerns, it's usually in the same conversation as employee pay, and the rep and company negotiate in that context to provide a solution for both issues.
The most safety concerns the union rep can cite - the greater leverage they have in salary negotiations.
This is why when a union says they've been bringing up safety concerns, you have to take it with a grain of salt.
...this issue is also why the government has moved most safety inspection responsibilities away from unions and towards government agencies, like OSHA.