"Adjustable hours" is not true everywhere, and being expected to work large amounts of unpaid overtime is extraordinarily common in this industry. There's many with companies with forced arbitration clauses for disputes and other unfair practices. Unions provide a great deal of protection against these abuses for pretty insignificant union dues.
Workers have more power now due to the red-hot tech job market. This will not be true forever.
I like to think of unions a little bit like getting a vaccination. You may not need it now, or ever, and when it actually works for you, you may not be aware of it. It's a low cost way of protecting yourself from injury and loss. And, there's a bunch of people out there that are really really convinced they are terrible for you.
I'm completely okay with underperforming software engineers getting fired. I don't want some system that I'm paying for with my union dues helping them maintain a job that they don't deserve.
On the other hand, I've never seen a deserving and talented software engineer out of a job for very long. And if you're talented you will get paid much much more than average.
The only software engineers that want or need a union are the ones that need artificial barriers to keep them employed.
Workers have more power now due to the red-hot tech job market. This will not be true forever.