PoS chains face a subtle problem PoW does not: a validator who has been offline for a long time cannot tell which chain is canonical by looking at work alone. This is weak subjectivity — the requirement that a syncing node obtain a recent trusted checkpoint (a weak subjectivity period) from a peer, block explorer, or community source before it can safely follow the chain.

Why does this matter? In PoW, the heaviest chain is unambiguous from genesis. In PoS, an attacker who once held a majority of stake could theoretically build an alternative history from an old checkpoint. The defense is social: nodes must not revert further than the weak subjectivity period, roughly the time it takes for 2/3 of validators to exit.

Sync from genesis: safe in PoW, unsafe in PoS without a checkpoint.
Weak subjectivity period: on Ethereum, roughly 2–3 weeks depending on validator churn.
Long-range attack: the specific threat weak subjectivity defends against.

Attack	PoW defense	PoS defense
Long-range	Not applicable	Weak subjectivity checkpoint
51% reorg	Hashrate cost	Slashing + finality
Nothing-at-stake	Not applicable	Slashing conditions