A pro is the work environment is much more honest, you are free to speak your grievances about management in front of everybody without worrying about them trying to fire you or scheme to make your life hell. You will have employees elected as shop stewards to handle all employee disputes instead of bringing in the heavy hand of management to deal with petty personal issues. Anytime you are summoned to a meeting room for discipline you get to bring a representative, and you get to appeal every single bullshit disciplinary action they take and almost always win, managers have no clue of the collective agreement.
Your job also changes to being a contractor, as you are contracted to do specific things which of course means if you finish them early, you can leave and still get paid 8hrs. You can also refuse work you have not been contracted for, and this is of course the major complaint about union employees how 'they're lazy and won't do what I ask' but if you asked the plumber you contracted to install a dishwasher to also mow your lawn for free they wouldn't do it either. That's how a collective agreement works, you have entered into an agreement to do X, unless there are special provisions for Y you don't do it. One of my collective agreements has provisions for extra work, and every day we push software on time in a research lab without some manager hovering around.
But it's not free, you just said they're paying you for at least 8 hours no matter what. Sounds like you're saying certain things are below that plumber and he won't do them, but still wants to get paid.
Edit. There is one case where I have seen benefits. My brother in law is a heavy machinery operator and works similar to a contractor does in tech. He goes in, does some work til the job is done, and leaves. He likes that he can get jobs through the union and that his wages were way higher in the last recession.
I have dealt with unions as a leader, I have dealt with a union trying to unionize employees that worked in my teams and I have dealt with union based employees in a plant and non-unionized employees in a different plant within the same company. Guess who got better benefits and pay? Wasn't the union employees, too much money had to go to pay the union dues, legal fees and unnecessarily inflated benefits, so we would fight every penny and ask on that side. And yes some states in the U.S. have rules around union employees can't be paid less than non-union doing similar jobs, but trust me large corporations know how to work those rules properly.
While I have lots of opinions based on my experiences, in the end what I learned is that unions are honestly not there for the employee, they are there to raise employee wages/benefits so that the union can collect more in fees (generally a percentage of total compensation, so salary + benefits). The sad fact is they honestly don't care about you, your family or making it better, they get paid more by making the business pay more wages whether the market and work demand it or not.
There was a good reason unions existed in the past, and I am a fan for their reasoning at the time, not today. They are now nothing more than a big business putting their hands in your pocket to take money from you. By the way, did you know in many states, if your fellow employees unionize but you opt out that you can still be forced to pay union dues? Yea, this has been litigated in the recent past and some states changed that law, but just beware of what you ask for or support.
Without a counterforce it's just a matter of time until we regress to ever worsening conditions for workers. Capitalists never gave anything away voluntarily. We need unions or other workplace regulations to keep things in balance.
Today in the US there are at least 4-5 agencies that oversee companies and their behavior. Laws are now on the books to protect workers. Workers can now sue their employers for violating their rights. Workers have protection of whistle blower laws. Are any of these perfect, no, but these are the true reasons unions existed. It was to protect the worker from abhorrent conditions and treatment. That has happened through the new laws and agencies in place. What we should be doing is forcing the agencies and politicians to use the tax money they took from us to clean up the areas which are not good enough still, not giving away more money to a union which is just another business there to take their share of your earnings and the companies earnings. Unions are a tax on everyone, they are not free, so you are taxed twice for 90% of the same protections, and the other 10% I would argue are not protections but limits on your rights.
I still see places we need it to be better, but that doesn't mean tech is a place for unions. Can you imagine all these 2-3 year post grad engineers getting the title of Senior software engineer and getting the equivalent high salaries or out of turn bonuses for great work now learn they have to wait 20 years to be called senior, can never again get an out of turn bonus or special treatment for going the extra mile. Unions equalize by taking everyone down to the lowest level of commonality, you are no longer an individual you are a cog to be used to further the union leaderships goals, not yours. Sure, you get a few extras for that but I for one don't want to hand someone else my freedom to earn and advance at my pace so they can get what they feel is important.
* edited a few words to make it clearer.
I don't expect it would happen at my company or at others as the job market for software developers in London is excellent; typically if people don't like the work they just leave.
But in general, sure. I think it's good for employees to organise and be able to present themselves in a united manner to the concentrated power of corporations.