They can be good or bad, just like anything run by human beings. Historically, Unions have been instrumental in raising working standards in much of the civilized world. Who knows how long it would have taken otherwise? Also, historically, there have been unions that were little more than criminal organizations.
As far as your example about McDonald's, that kind of ignoring half the picture. Big corporations work EXTREMELY hard to keep unions from forming. Get a job with someone like Walmart and they'll bombard you with anti-union propaganda. Many retail chains will make you watch an anti-union video and urge you to report any nefarious looking union reps. However, whether they unionize or not, the THREAT of unionization is very helpful when it comes to corporations treating employees fairly. If they didn't have that possibility, I bet things wouldn't be as peachy as they are now.
That being said, unions have been part of some things that turned out horribly, though the business world is just as responsible. Some of the pension plans and other concessions won by unions previously have literally doomed corporations and brought certain industries to their knees.
