the disadvantage of a single party state is that abuses within the party cannot be easily challenged, whereas in a multiparty state there is always someone with an incentive to expose abuses and who hopefully has the power to punish the perpetrator.
a similar critique applies to a monolithic religious hierarchy, which tends to be rife with abuse unless it is embedded in a secular society that exercises effective oversight into its operations.
similar problems affect societies with no social safety net, or company towns with only one major employer, whose abuses cannot be easily challenged given the lack of alternatives.
if a person has no means of escape or recourse then you basically have to assume they’re going to get abused as you’ve already conceded that there is no way to ensure that they aren’t or get redress if they are.
the same logic applies to schools, prisons, families, companies, towns, every collection of people.