They can use your authorization card to impose their representation on you without you ever having a vote on whether you want the UAW to represent you.
They can and do make many promises about what they will get for workers, but these claims are often empty promises to get workers to sign up.
Membership requires payment of monthly dues to the Union.
Members can be put on trial and disciplined or fined for violating UAW rules.
They can make it harder to reward and promote high-achieving workers.
They can drive up costs, leading to strained relationships between labor and management.