Unions can't stop layoffs - at best they can just change the order in which employees are laid off (First-in Last-out / seniority), I guess the process might make the company reconsider, but there's no direct intervention.
I'm a member of a union and went through layoffs.
I think it'd only change if unions had power on the board / guaranteed share ownership, etc. as is the case in Germany (board representation with workers' councils) and was proposed but rejected in Sweden (share ownership).