https://en.wikipedia.org/wiki/Proof-of-stake#Criticism

These problems seem fairly significant and unresolved. Thoughts?

Statistical simulations have shown that simultaneous forging on several chains is possible, even profitable. But Proof of Stake advocates believe most described attack scenarios are impossible or so unpredictable that they are only theoretical.

Any system that will eventually attain a $1T market cap probably can't have theoretical issues like this.