I support unionising in general but not for contractors. By definition contractors have agreed to do short term work according to a specific contract. Trying to negotiate the contract later seems dishonest. That’s a very different to a regular long-term open-ended employment contract where unionising makes sense.
The “contractors” are W-2 employees, they’re employed by an external agency rather than by Google itself. This agency then contracts to provide services to Google. It’s a bit like if a building outsources their janitorial to A1 Cleaning Services Corp, and then A1’s employees unionize. The thing is then it might be legal for the building to cancel the A1 contract, and then for A1 to fire all their employees due to the lost contract, and then for the building to hire A2 to do the same thing. This is a loophole in union busting laws.