There are concerns. I have expressed those concerns in some of the work that I've done. What you have here is a very exceptional context when arbitration is used to resolve some of the most sensitive public policy and public law issues that can arise in any country.
Those issues are normally resolved in courts. Courts enjoy institutional safeguards of independence, such as security of tenure, objective methods of appointment of the judge to individual cases, and prohibitions on the judge earning income on the side or working in an overlapping role as both a judge and a lawyer in different cases. All of these concerns arise because of the unique context in which arbitration is used here, and it does give rise to serious concern.
I never allege actual bias because I don't think that's necessarily a decent thing to do. I don't wish to cast aspersions on any individuals, but that's not actually the key concern. The key concern is that everyone involved, investors, states, and so on, be confident that this is a fair and independent process. When you lose the institutional safeguards, you give rise to reasonable apprehension of bias, and that's the problem.