Using the proxmox kernel has nothing to do with running VMs. The original reason for adding it was that it was stable version (backports is usually a moving target) and the zfs module was built-in eliminating the problematic compiling. As for being less battle tested, I disagree. The proxmox kernel is a minimally modified Ubuntu kernel (very stable on the hundreds of Ubuntu systems I maintain in the enterprise). And since Proxmox is using a Debian userland, running the Proxmox kernel on OMV is a perfect fit. I have been doing it for years on many systems including production systems. The Proxmox kernel also offers better hardware support than the standard Debian kernel. You can use whatever kernel you like but that is your preference. I don't think I have ever seen the proxmox kernel cause a problem.
I'm glad to hear you haven't had any issues, I'm just quoting what proxmox staff themselves have stated.
As for running VM, the research I've read indicates it's optimized for VMs. In fact, that seems to be proxmox's "thing".
Their own github even indicates various settings configured to help with VMs: https://github.com/proxmox/pve-kernel.
I ran Gentoo for over a decade, I've been jaded by system breakages, and want to diverge as little from the baseline system as possible. This means default debian kernel, minimal community maintained packages, that sort of thing.
I don't care about "bleeding" edge, as far as it goes here. I just want it to work.
Anyway, I appreciate your feedback on it.