I have made a comparison in recent weeks between proxmox and xcp-np/Xen Orchestra and for me proxmox is not mature enough for a work in production considering different aspects. Xcp-ng, if I see it as a solid option, especially if you pay for the Xen Orquestra subscription, which in addition to unleashing the integral management of your entire xcp-ng park, also allows you to make backups
Nutanix is not especially cheap, in my opinion/experience, nor is it particularly easy to manage and maintain