It is contentious because legal research has repeatedly demonstrated that there are legal fields in which for an undetermined reason the plaintiff faces far longer odds than they would have in court.
This concerns arbitrations that in 90% of cases are in favor of the defendant. While in 'normal' case law you see rates closer to 60%. (Yes the research also accounted for cost differences by comparing similar classes of cases, no-cure-no-pay etc..)
It is also suspected that the plaintif usually gets less compensation for damages through arbitration.
Another issue is that arbitrators sometimes earn a lot money and are obviously incentivised to attract 'customers'. The problem is that there are and will always be many more plaintiffs than there are defendants. So the selection of arbitrators depends on decisions by the future-defendants. (Unless a specific organisation is legally mandated, but I've not encountered this before)
Lastly: I'm not a corporate lawyer, so I don't know what legal reasons a company has to prefer arbitration over legal procedures. But they must have some. And those reasons are unlikely to lead to worse outcomes for the company.