The union takes away judgment by merit and replaces it with seniority or loyalty to union management. How hard you work, how well you do your job, how qualified you are - all cease to matter. It's only how long you've worked there and whether the union bosses like you.
The employers become antagonistic to employees because unions make it an us versus them situation. Unions get higher wages by blackmailing businesses, give us more money or you won't make any money at all. Workers give up the higher wages to the union, making it a wash, in exchange for losing out on performance pay. Then businesses lose good, young employees if they need to cut staff, even over the bad apples who've managed to be protected by union bosses.
And when unions get too greedy and demand unsustainable wages and benefits (think UAW), businesses move work overseas. And the employees lose their jobs. And union bosses keep their pensions.
Don't give up the right to be paid what you're worth. Or willingly have a work boss AND a union boss to report to.