I personally have raised concerns. In the literature, in the discussion about it in the field, it's usually called the club. The arbitrators come from the club of arbitrators. I think it's fair to say they appear to have an interest in favouring claimants, because if claimants don't bring more claims, they're not going to get more appointments. They get paid by the day or the hour, per appointment. They're not like courts with judges who have secure tenure and have a guaranteed salary from the state, and so on.
Yes, you always have to wonder if the arbitrators will be thinking about who has the power to appoint them in future cases. When you give executive officials the power to appoint the judge case by case, that just doesn't satisfy basic requirements of judicial independence that we've recognized for a long time, at least in a common law context.