The security of AWS PrivateLink relies on three factors: the path, the policies, and mode of communication.
The path between a VPC endpoint and a service stays within AWS and does not traverse the Internet. It therefore remains out of reach of Internet breaches.
When you are using VPC endpoints with AWS services, you can also create endpoint policies, which restrict access to requests that come to the VPC endpoint.
PrivateLink does not provide any encryption by default for data in transit. The service consumer always initiates the service (it is a one-way service), and that the service provider only provides service to allowlisted customers.