There's power in bargaining as a collective, whether you're a collective of ten or ten thousand. Either way, it helps with the power imbalance between employees and management.
I'm reading this as a sign of cultural stress unless we have evidence to the contrary. (EDIT: I didn't realise Fastmail has almost 1,000 employees, with many remote. That comes closer to where structure is merited, though I'd still argue that this points to dysfunction in the U.S.-HQ bridge given only the Americans are unionizing.)
Take this to an extreme: a start-up. A union is overly bureaucratic. I’d wager a union doesn’t start making sense until the lowest-ranking employee ceases to have on-demand access to senior management.
Sure. But if you had a grievance, you could voice it. And if you needed to pull some like-minded coworkers together to underline it's shared, that could be done ad hoc. If a 50-person company needs a union (because e.g. management refuses to listen) that's a problem. That's my point.
I don't know where the delineation is. But there is very obviously a point below which unionization is a sign of dysfunction. For the same reasons a middle-management layer or expansive C suite, below a company of a certain size, is a sign of dysfunction [1].
FWIW I don't think that's obvious at all. Even a small group of happy employees could form a union to set the current policies everyone is happy with in stone and protect themselves and future employees against potential changes in ownership or a downturn in company health.
I'm not arguing it couldn't be done, nor that it doesn't have benefits. Just that it has costs, and those costs at a small level should outweigh the benefits. While they're doing that, and maintaining that structure, they could be doing something else. (Something more enjoyable and lucrative.)