The main reason for the fear is the loss of class action potential, which means almost all small disputes are impossible to obtain redress for. If you're in a situation where the case is worth 5-50k, it's generally a better option to go for an arbitration barring concerns associated with bias (which very much do exist - many specialty boards like various copyright boards consistently make pro-industry mistakes in procedure), but the primary overarching anti-consumer element of mandated arbitration clauses is the removal of class actions as a vehicle for making mass-litigation viable.