It is not clear whether these employees are actually in a good position for negotiating. The idea behind unions is that an employer is not willing to lay off all the employees that are unionized (because this would lead to a sharp decline in productivity and thus KPIs). I consider how many products were scrapped by Google as quite some evidence that Google would be nearly as successful if it fired the unionized employees and continued working with some "core team".
This does, of course, not mean that I endorse this reality, but when you negotiate, you better know what leverage you actually have.