This is also technically true - except your union is going to collectively bargain a binding contract which gets you all those things, and prevents you from being exploited or the employer from randomly changing rules to exploit you.
You have no guarantees on pay, benefits or work rules without unions either. The company can change those at any time or never change them at all.
At least a union will fight for those things on your behalf. A company has no incentive to do so and will actively oppose such things if it hurts their bottom line even slightly.
Correct. Benefits can and do get cut frequently without unions. Benefits cannot be cut in a union under contract, and if they try to cut them on the next contract you have the power to collectively bargain and strike if they do not come to the table and bargain in good faith. The recent WGA strike is an excellent example of all of that.