The limited access to staking results in centralization and loss of network resilience to attacks and governance manipulation.
Limited node number or diversity causes low physical resilience of the network. It can lead to concentration in the hands of a few early network participants who can collude to mount attacks or maximize their profits through on-chain governance.
Insufficient staked capital leads to low economic resilience. It makes the network attacks relatively inexpensive and can lead to double-spending or market manipulations. Emerging blockchains are especially vulnerable, as they operate with few nodes and tokens staked.
Other security issues may stem from difficulties accessing PoS governance, which promotes the interests of a small percentage of voters.
Specific PoS protocols, i.e., DPoS (Delegated proof-of-stake) blockchains, have been criticized for promoting oligopoly and collusion by limiting the number of validators.