Some sites have SSO and some don't, I am currently using Proxmox in a Ceph deployment for the Navy and we are using an SSO (keycloak) for Smartcard/session timeout support, but it was still a tough sell to DoD security. Unfortunately a lot of security/IA types assess a set of checkboxes and either they align to the standard or not, they tend to be light on the technical side. Across the street at a different Navy command they won't touch Proxmox because it won't meet security requirements.
The big guys like VMWare just get the DoD STIG and the law enforcement/medical/intel equivalents and natively support all the findings. Smaller groups like Proxmox have to pick and choose. I think the banner (pre login) and Session timeout are low hanging fruit that for some reason get the Security guys all excited.
What I love to use Proxmox for is small deployments and exercises that require a fast turnaround, it so superior to VMWare/RHEV/etc for that use case, but without an SSO infrastructure it gets slammed by security sometimes. We can get VM management, containers, and storage from one tool and it is easy enough to use and document that people with limited IT skills can manage it where as straight KVM + Docker would require an extra person(s) to be deployed.