ERC721 by itself will not make a difference in creating a truly unique decentralized identifier. The only thing that can make an identity unique is consensus that it is unique. ERC725 is by itself not an identity, but a key management and proxy contract. With the addition of attestations via ERC735 you can build up an identity based on the strength and number of attestations and consensus about what they mean. Different parties may more or less trust the identity to be unique based on the attestations tied to it.
For example, if your country / state can reliably issue a claim that is tied to your ERC725 address then you could use that to prove you were a unique individual to most people. In order to verify that uniqueness parties that care about it need to verify the claims attached and make a judgement based on the claims. This would apply to ERC721 as well, you would need attestations tied to the ERC721 token for it to act as an identity.