iirc the pow difficulty was like a perfect source of randomness, they have removed that now, breaking some smart contracts, but they can be updated, and also setting a precedent for every smartcontract needing to be upgradable i guess, to the new randomness feature RANDAO, which is not a s good as the PoW randomness.

They also changed the blocktimes, so for example smart contracts that calculate interest based on blocks would have to change too. So i guess more things will break during this transition period