When the ETH becomes pure PoS, it is more difficult to fork. That is according to the comments by the Vitalik himself.
What kind of POS have ETH ?
Probably it will be hard when most coins are on exchanges hahaha... and voting to change protocol will be will of
exchange owners today probably poloniex will be king.
I think that can be ETH issue. Polo was not hacked for long.
POS have problems with that matter people don't realy care about stacking coins for 1$ revenue while it cost 120$ year for power.
And we and in world where a bigger stacker will be exchanges - always online and big chunk of coins in their hand.