Scale could be recommended as a hypervisor *particularly* in the context maturity of platform
Ah, I see your confusion.
Scale and Proxmox both use KVM as the actual hypervisor, and ZFS for storage.
It helps to understand the history of hypervisors. I don't have anything particularly thorough written up, but one of my clients is an OnApp customer and I remember this article pretty well:
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It describes the evolution of both Xen and KVM in some detail, and provides some comparison and contrast. Unfortunately it does not include ESXi.
KVM is a type 2 hypervisor that has in perhaps the last five years started to make significant headway and pull ahead of Xen, previously considered by many to be the "second place" winner in the hypervisor wars, behind ESXi. However, both Xen and ESXi are "type 1". For arguable values of what that means.
Now, your confusion probably stems from the fact that while Scale hasn't even seen a release, this must mean that the hypervisor capabilities must be equally untested and questionable. However, since KVM is mature, just as ZFS is mature, these underlying technologies can be acknowledged to be relatively stable and known qualities. I wouldn't be afraid of Scale or Proxmox losing a ZFS pool, for example. They're all using the same underlying code.
I am fine with conceding that Proxmox has more polished VM handling, as it was designed to be a hypervisor platform.
However, Proxmox lacks filesharing capabilities. Let's give Proxmox a 10/10 for virtualization and a 0/10 for filesharing. Total score, 10/20.
So, Scale has impressively powerful and well-polished multiprotocol filesharing capabilities, but kinda weedy support on the VM front. There's nothing particularly dangerous about the VM support, since it is based on KVM, but it is more clumsy and less complete than Proxmox. Let's give Scale a 10/10 for filesharing and a 4/10 for virtualization. Total score, 14/20.
Scale wins.
Oh, I didn't give Proxmox any credit because you can do filesharing as a VM on top? Well, fine. However, we've had enough people come through here with Proxmox issues that I could no longer give it a 10/10; charitably I'll give it an 8/10 for virtualization, and I'll give Scale as a VM only a 4/10 because of all the wasted space and poor performance. Layering ZFS-on-top-of-ZFS is going to be a bad thing, and yet what a lot of users will want to do. That's still a total score of 12/20.
So, in the context of the OP's question, I feel it's a reasonable answer.