It is potentially a good sign if Fastmail management voluntarily recognizes the union - that would imply that management and employees are on relatively good terms and the union is not asking for enormous changes.
Even if you love your company, love how you're treated, and don't want anything to change, forming a union still gives you two big advantages. First, it gives employees more influence and voice over the future of the organization, and second it protects the current conditions you love so much and makes it much more difficult for management (which can change at any time) to take them away in the future.
Here's one illustrative example: the staff of Politico and E&E News announce they are unionizing (https://newsguild.org/staff-of-politico-ee-news-are-unionizi...). Why?
> “We want to establish certainty and codify all of the things that we like about E&E into this new world order,” said reporter Emma Dumain. “E&E News is the most family-friendly newsroom I’ve ever worked in,” she said. “I feel supported by my editors as a working parent and that support helps me be a better journalist.” She wants to make sure everyone has the same support, no matter who their editors are or who owns the company.
> Leah Nylen, who has been involved in four company sales over 15 years, said that each one led to changes in benefits, bosses and sometimes layoffs. The day the Politico sale to Axel Springer closed, management informed workers that because of an oversight their 401K would be frozen for the rest of 2021. “If employees had had a seat at the table, I doubt something as important as our retirement and healthcare savings would have been missed,” Nylen said.
Less than two weeks later, management voluntarily recognizes the union: https://newsguild.org/politico-ee-staffers-win-union-recogni.... Everyone seems pretty happy.
> With union recognition nailed down, Organizing Committee members immediately began informing coworkers about the new rights that brings. A legal protection known as “status quo” requires management to maintain the same wages, hours and working conditions that existed when the union was formed, unless they negotiate an agreement with the union or reach a final contract. “Just a week-and-a-half after going public, we have all of these new protections,” Snyder said.
How's it going two years later? Well, not great honestly, they still haven't agreed on a contract, but Politico is still going strong and I imagine most people there are still happy with their decision to unionize.
Here's an article about voluntary union recognition: https://www.americanprogress.org/article/voluntary-recogniti.... This is not an unbiased source, but you can ignore the framing if you want and just use it as a big collection of links to voluntary union recognition examples.