To fully understand why unions are bad, in general, you have to have a pretty solid understanding of business, economics and international trade/competition. Getting into the finer points of all that would take too long....those are college/graduate courses not internet postings....So in a sense, my efforts here will probably be futile, but I'll take a relatively brief stab at it anyway
Unions hurt the middle class. To use an off the cuff analogy, they hurt the middle class in the same way eating shitty food and being sedintary hurts fat people (or anyone for that matter). Sure it's easy and it's comfortable, but you get used to it and then you get to the point where you need it and can't change your ways.
Pro union people love to point to "this country was built on unions" and while that's not incorrect, the sentiment is misleading IMO. This country was built in a rapidly expanding environment in a vacuum, void of international competition. Until the last 30 years or the only real industrial nations were in Europe, which was war ravaged twice in 25 years during a time when lots of the US development was going on. So the US, with it's relatively pro business policies (everything is relative, remember that...ex: the dollar is strong right now, not because the dollar is fundamentally sound, but because everything else is a steaming pile of dog shit). Those pro business policies, along with a rapidly expanding country (booming population and plenty of room and resources to fuel that growth) allowed our country to excel. We were among the first to industrialize and weren't war ravaged and were able to produce and export goods to the rest of the world who was trying to catch up. The rest of the world posed little to no compeition to the American industrial machine....they just couldn't match our sophistication in anything...also remember that 1/2 of the world's population was mired in communism...shut off from the rest of the world. The US was a lone, shining beacon of capitalism and because of that no one could touch us...profits were easy....THAT IS WHY UNIONS COULD WORK. It was a two party negotiation between labor and management (it was still inefficient by the way, but not as devastating). Labor wanted things and that could drive the price up, but all that meant is that the price would be passed along to the customer and maybe a few less widgets would sell and the company wouldn't make as much, but would still be fine since they likely only had competition from other American companies (or european) who had union issues as well...so their costs were the same.
But as we all know....that's a far cry from the world we live in NOW
Because the US was so much better than the rest of the world for so long its industrial processes stagnated (that's why the "Made in the US" label had become such a joke). US companies didn't change because they didn't feel they needed to change. They had no competition, could afford their bloated union employees and made the same stuff with little improvement over the years. Without competition they were lazy and ill equiped to handle the foreign competition when it came...intially from Japan (a country with no resources who was bombed into oblivion, forced to be efficient or parrish and when they finally came around they had quality items that were cheap). With the competition from abroad, the US had to adjust, but they'd been so fat and happy that they struggled to do so all the while, other industries from other nations were catching up. It's like the kid who is 6'2 in the 6th grade....sure he's the king of the basketball court now, but if he doesn't get any taller or any better, eventually the other kids are going to grow and be able to take him down.
First we had Japan, then India, China, Southeast Asia, Russia, Mexico and eventually Africa or some other undeveloped place like that that will want their piece of the pie. Now we have international competition from countries who don't have "unions". They have developed to where their workforce is marginally educated and can give the American middle class a run for their money as far as productivity, yet because they live in developing countries (who don't have unions) they will work for less. All of a sudden those bloated unionized industries don't have the same cost structures as their international competition and the competition can price them out of business. Sure they may be able to limp along for a while, but why would you do that? If you have to pay the American unionized worker $20/hour but the chinese man will do the exact same thing for $3/hour and you're in a labor intensive industry you have to move to china to stay competitive....you just HAVE TO....it doesn't make any sense to do otherwise.
Unions, in general, destroy an organizations flexibility and in this global marketplace you have to be flexible to stay competitive.
Personally, I think unions are the cancer of the middle class. I'm sure many of you will vehemently disagree here (because it sounds like a contradictory statement).....People in unions might love them because they "help provide for their family" or some such line like that, and that's not untrue on the surface, but if you look at the real effect it's much darker. The days of graduating high school, getting a union card and being set for middle class life is over. The developing world has made sure of that (not "greedy" fat cats/corporation as some of you would love to think). Look at folks within a union....they've been doing the same shit for years and probably don't know how to do anything else....they've been too comfortable and likely haven't bettered themselves (professionally) in any real way so when layoffs happen, (and sooner or later it's going to happen) they are woefully unprepared...probably have a lifestyle that their skillset can't afford and are in a heap of trouble unless they can find another of the fewer and fewer union jobs that are available.
I understand why the working man wants to unionize, but it's short run gain and it's just not sustainable in the world we live in. You can't expect to just graduate high school, be a diligent worker for a company and live a comfortable life in the most economically advanced society in the world we live in today....there are just too many people globally that want what you have and will work to get it.
I don't even want to get into the finer points of trying to manage unionized labor and how disasterous that can be in and effort to effectively run a production floor.
There are many other points and reasons, but this is just a brief (it's very brief, despite it's relative length) overview of why unions don't work.