Here are some corrections:

• Blockchain has nothing to do with P2P.

Blockchains are federated ledgers that can’t be changed later, unless the majority of federated servers decides to. P2P means that two devices communicate without a server in the middle. Maybe you meant federated?

• Blockchains can absolutely be hacked. You can gain majority control over the servers, in which case you can rewrite the blockchain as you want. Alternatively you can gain access to accounts/wallets by hacking the software that users store them in or by social-engineering people to give you their keys.

• If proper end-to-end encryption is used, there is little security difference between server-based and P2P communication, but it’s much more inconvenient: You cannot save sent messages on the server for later retrieval, so if you’re trying to reach someone who’s currently offline, your device has to wait until they’re back before sending the message. Also if you use multiple devices, keeping them in sync is very complicated, because they have to be online at the same time.