So - a union (as plenty of people in this thread have pointed out) is a worker organization, intended to support and protect the (collective) interests of the workers in a particular trade or a particular industry. They may fail to represent any particular worker whose interests don't align well with their fellow workers as a whole because of unusual circumstances. And it's possible for a union to fail to represent its members as a whole -- this normally happens with big trade unions where the leadership and payed administrators get too cozy with management and too far removed from the shop floor, a structure encouraged by US labor law. However, unions are, at least in principle, On Your Side and representing Your Interests -- when they fail to do so, it is for the same reasons that representative democracy tends to fail.
On the other hand, you have management, who are out to maximize profits, especially short-term profits, and see labor as a cost-center to be minimized as much as possible. Management are inherently in opposition to Your Interests as a worker. And their opposition to unions is mainly due to the fact that the union gives you the power to fight for Your Interests in opposition to Their Interests.
So you see why the two are not at all equivalent? Pro-union propaganda may be to some extent false -- to the extent that it presents to you an overly rosy picture or a best-case-scenario that you are unlikely to realize. But anti-union propaganda is guaranteed to be an absolute fabric of lies trying to convince you to act against what are almost certainly your best interests.