I consider myself liberal, and generally in favor of things such as unions, workers'-rights, and anti-discrimination regulations, that serve to balance power in our society.
That said, I can definitely see an argument that unions are inefficient - they slow down rate of change, introduce barriers to new innovation, and require one-size-fits-all negotiation. This doesn't mean we need to remove unions, just recognize the tradeoffs.
Furthermore, I would myself argue that unions are a response (and a rational one) to an inefficient, asymmetric power structure. i.e., unions are a symptom of a problem, not a problem.
example:
Let's say there's an industry, where in a competitive, open, and rights-respecting market, workers could typically get $15/hr.
If there's a monopoly-employer and un-unioned employees, they can suppress the wage to $10/hr, concentrating profits to the employer. If there's a union, and no monopoly of employers, the union can force the wage to $20/hr, stifling the industry, and possibly preventing new employees from entering the marketplace freely. If the employers and union are at equal power levels, the wage sits around $15/hr, but there's a lot of red tape, and slowed rate of innovation, because of the nature of negotiations.
If you create a company that hires non-union workers, but treats them as well or better as unioned workers would be treated, you should have a competitive advantage.