Proxmox isn't really its own hypervisor. It combines a few common projects to make a OS. It is pretty much KVM with corosync for clustering.
With that being said it is a solid platform. Just keep in mind it is just standard Linux virtualization and for single nodes you can get the exact same setup easily on any Linux system.
Well, the exact same except for the frontend. It's arguably better than virt-manager imo. I wonder how hard it would be to get pve-manager running outside the OS.