I certainly agree with your statement. I grew up in an automobile industry city. At one time, one of the big three American automakers employed over 30,000 people just in my hometown. As times became more difficult, plants closed and people lost their jobs, but most from my hometown agree that the unemployment is not solely the result of tough times. While there was certainly a need for trade unions in the 1940s/50s, the unions (especially the UAW in this case) eventually forced companies to pay unskilled workers such high wages and offer unbelievably good benefits that the companies cannot sustain those costs while still makinga profit. In the 90s when I was in high school, many of my classmates' fathers worked in the car factories as assembly line workers and made an annual salary of $70,000--that's an incredible amount of money for someone with only a high school diploma! We witnessed last year that GM is virtually powerless to make the necessary changes that they desperately need to make in order to turn a profit. Instead of looking out for their average union members, the UAW bargained and fought against changes that would truly keep more jobs open and put the company back on track.
In addition to the specific example above, trade unions have also hurt the modern worker by:
--strongarming them to vote for specific candidates or parties (ones who agree with union doctrine)
--forcing workers to lose pay and work time over issues that the workers might not have individually chosen to strike over.
--causing companies to move their plants/branches to other states or even other countries where unions do not have control.
This can only be an opinion -- there is not one factual answer. The answer will also surely vary depending on what country you are talking about.
In the developed world, you can argue that unions have become a curse to workers because they demand too much. You can say that the unions are asking for too much pay and too many benefits. This ends up being bad for workers because it hurts the companies so much.
For example, you can say that the American car companies' problems came about because their unions were too strong. The firms became uncompetitive because they had to pay too much to current and even retired workers.
If unions' demands destroy their members jobs, they become a curse.