This is still compensation, if they are being paid by the company for what the company is willing to offer, it would need to be paired with a cash reduction, or amount to a raise paid for by the company.
Unions typically already have a seniority over competency problems, this seems like something that would only really exacerbate the issue.
You also have to consider these are warehouse workers not software developers, there is a very high turn over rate, I don't think many of the warehouse workers would be interested in a vesting period before they saw anything from their RSU.
I also and not sure how this would work in a PLA multi-union site, lets not forget that the truck drivers, electricians, and many others already have their unions, and in my experience once a place has a labor union onsite, they want supporting employees to be union also.
And I think we will have to agree to disagree on conflict of interest.