The difference for PoW is that it is costly to even try to do such an attack. For PoS, there's only the initial cost of the coins. There's no running cost for tries, retries, etc.
...or the effort taken to steal the coins, or the effort taken to borrow the coins(bank, payment processor, exchange, ponzi, ect..), or the effort made to compromise the wallet of a whale, or the effort made to take many loans out....
Some of the above can be applied to PoW as well, but if you compromise a mining pool operator and you use their miners than there will be an instant reaction and correction to stop such an attack from re-occurring or happening in the first place because there is large amounts of investments and expenses to be paid in mining.
With an attack on a PoS , the transactional history is tainted and any rollbacks ruin the fungibility or anonymity of the currency.
economical cost while attacking PoS does not.