The case can be disciplinary in nature such as member's appeal of a disciplinary action by the employer. Or the arbitration can be over the interpretation and application of the agreement where an individual is not a granted chance to work overtime or given a promotion because the employer has violated the seniority clause of the contract. Witnesses are questioned and evidence is presented. In an expedited process, the decision may be given right away. The arguments made against the arbitration process are pretty well-known.
Arbitration is time consuming.
It may be a year or two to go through the grievance procedure and get a case heard by an arbitrator. In the meantime, the member is waiting for justice. Arbitration is an expensive process both in actual money and staff time. These costs can quickly bankrupt a union treasury and employers know it. We also may not like what we hear.