Susceptibility to length extension would also have disqualified SHA2-512 from SHA-3, where that property was a requirement, so it seems like the cryptographic community has come to conclusion about this.
The "security margin" of a full SHA2-512 digest, over its truncated SHA2-512/256 alternative, is not meaningful in practice.
If you want to use full-width SHA2-512, go ahead. SHA2-512/256 is safer.
The "security margin" of a full SHA2-512 digest, over its truncated SHA2-512/256 alternative, is not meaningful in practice.
If you want to use full-width SHA2-512, go ahead. SHA2-512/256 is safer.