KVM is awesome enough that there isn’t a lot of room left to differentiate at the hypervisor level. Now the problem is dealing with thousands of the things, so it’s the management layer where the product space is competing.
Libvirt is the abstraction API that mostly hides the concrete implementation details.
I haven't tried oVirt or the other UIs on top of libvirt, but it seems less painful to me than digging through the Proxmox Perl modules when I hit a limitation of their system, but most people may not.
All of those UI's have to make sacrifices to be usable, I just miss the full power of libvirt/qemu/kvm for placement and reduced latency, especially in the era of p vs e cores, dozen's of numa nodes etc...
I would argue for long lived machines, automation is the trick for dealing with 1000's of things, but I get that is not always true for others use-cases.
I think some people may be supprised by just targeting libvirt vs looking for some web-ui.