Consistency of single updates is vastly different than multi-write atomic transactions. The former precludes, for example, financial applications which require atomic updates of multiple balances.
Financial transactions are pretty much the poster child for atomic, multi update transactions and pessimistic locking.
It is not the transaction itself that is hard, it is the network partition. E.g. what happens if two network partition approve transactions, that wouldn't have been accepted if there were no partitions.
If this DB is the sole record of The Money, and I can move some quantity from X to Y in a transaction, then that's fine by me.