In companies I've worked for, union facilities have far higher fixed conversion costs that weigh down profit growth. They also make it extremely hard to be cost-competitive or price leaders in the market, because they oppose and sabotage investment in capital that improves speed and efficiency, which would reduce unit costs.
Additionally, they make retention of your best and brightest tough by forcing promotions and growth opportunities to follow seniority over performance.
They lose to non-union facilities and companies constantly.