I agree and disagree. For the average low paying job that doesn’t require any skill or knowledge, a union is the only arguing power you have. However, if you are hard to replace because of skill/knowledge and you know it, the power balances back. You have the power to say no without consequences.
Obviously don’t push it to where the cost of replacing you is worth it for your employer, but you have more power than most realize. Making yourself invaluable only increases your say in how things go. So getting the time you requested off is certainly doable.