Another thing I haven't seen mentioned here is that bosses will often try to slow down, stymie, or break up organizing drives by dragging out legal arguments at the labor board. This means arguing that the bargaining unit should be broken up, that certain employees are actually supervisors and shouldn't be included, that certain supervisors actually aren't and therefore _should_ be included in the vote, that the proposed bargaining unit is too small and should include more employees, etc. Typically you'll need a lawyer to navigate this process, lawyers are very expensive, and national/international unions have money.

Organizing a union is also fairly hard, and staff of established unions are more likely to have the skill set to teach you how to have convincing conversations with your co-workers, make sure they stick together, keep everybody from freaking out when the boss starts threatening to fire people, etc. It's also good to have them around to provide financial/legal support if the boss follows through on those threats.