If some factors are outside of of the trade union's control then it's also outside of the worker's control. Therefore, forming an union has no impact on the problem.
A trade union has in fact control over important aspects such as wages. The odds of a negotiation between a candidate and a HR representative benefiting the candidate are fare more favorable to the candidate if he has at his side someone who is informed about the company's hiring practices and salaries and has a vested interest in improving salaries paid by the company to his fellow colleagues. If a job seeker goes into a negotiation room alone then this information asymmetry alone is bound to screw him over during negotiations. The presence of a union representative is very effective at mitigating this problem.