Those unions aren't specific to one company/team though and are mainly to set pay floors because they work in a glamorous industry that people are desperate to get into and so they are used to raise the people outside of the competitive spotlight pay in the industry. If you want to start an non-company specific union for developers, by all means, start one but I see no reason to join it considering the only reason people are desperate to be developers is because it pays well.

What a software engineer union would be like depends on who you ask. Thus the risk as no one agrees and thus the result would be a crap shoot. Worse, it'd become a political situation so even if it starts aiming for A the movement may be hijacked to cause B. For example, many people mention preventing things such as unexpected PIPs and firings which sounds a lot more like teamsters than a guild.