In a small business, maybe you're the only person doing <critical task X>. This is a more level playing field, but maybe a union is still useful here also.
>In a small company, a real conversation is possible.
Why do you think employees have enough leverage across most SMBs to the extent that workers shouldn't organize?
Even SMB owners and managers organize adversarially against their workers (eg by setting compensation and benefits per VC or consultant provided guidance on labor market, by hiring HR specialists to coordinate with managers on keeping wages as low as the market will bear). Why should workers not in turn organize in those cases?
A real conversation is generally not possible anywhere. Your boss has enormous power over you and the reality is that even if you are somehow special, they won't see you that way. It sounds hopeless, but if you approach your boss in this manner (he couldn't care less whether or not you do actually get hit by a bus) the negotiation is easier to do.
In my experience, I have seen more power imbalance at small/medium sized businesses. Whereas larger companies you tend to get lost in the bureaucracy and have the ability to switch teams/orgs and get out of bad situations. HR is usually more helpful with this too.