- CBC needs padding, which when misused can lead to padding oracle attacks

- GCM allows for parallel encryption

> And whats wrong with AES256?

There are more modern, faster and better ciphers that are designed to not be vulnerable against many side-channel attacks that AES is difficult to protect against.

https://crypto.stackexchange.com/questions/2310/what-is-the-...

https://crypto.stackexchange.com/questions/14747/gcm-vs-ctrh...

https://security.stackexchange.com/questions/33569/why-do-yo...