Do you understand what it would need to change multiple blocks on 51% attack. Getting 51% of hashrate is a thing, maintaining long enough is another. Also if 51% attacks happen in a PoW consensus it can recover from it, not without any damage but it can recover. On PoS systems, once it happend it's done.
PoW has probabilistic finality. The deeper a block is buried under subsequent blocks, the less likely it is to be reversed. Deterministic finality is not a universal upgrade — it trades off with flexibility and recovery mechanisms.
Main risks regarding PoS resilience are in my opinion :
- Centralized staking: A few providers (Lido) control large shares of stake, increasing systemic risk.
- Recovery assumptions: PoS chains often rely on subjective knowledge of the canonical chain in recovery scenarios (see "weak subjectivity").
- Slashing risks: Honest validators can be penalized due to bugs or network issues.