The unintended consequence of the success of a blockchain is that it becomes more and more difficult to run a node of the blockchain.
Most of the transactions coming on the Ethereum blockchain come from 2 sources - Infura and Metamask node/s.
A denial of service attack on these nodes will stop transactions from being submitted and freeze DeFi and many other Dapp-s.
‘Oracles’ for Ethereum blockchain provide pricing information for trading.
Manipulating the input price which the oracle gets from centralized exchanges or the output they give to swap contracts will lead to liquidations of stakes.
Regarding Bitcoin - the bulk of transactions are submitted by a small number of wallet nodes of centralized exchanges.
In the same way one or more of these nodes may be attacked to stop or delay submission of transactions.
