How does something like proxmox compare to vmware in the larger space? What functionality is missing that is critical for larger businesses?
Genuinely curious.
So, these are a few things off the top of my head; I tend to limit my usage to only smaller installs, so consider this more of a "stuff to Google for clarification" list than a definitive set of information...
I think the biggest thing is that there is no analogue to vSAN. It'll mount iSCSI and NFS targets, and its ceph implementation is at least on par with VMFS, but larger installs that depend on vSAN tend to be underwhelmed.
The Proxmox Datacenter Manager, which allows for live migration of VMs between hosts, is still in an alpha state. I've had it work pretty well; it's quite polished for something being described as being in its alpha stage, but the functional equivalent of vMotion is still lacking.
Meanwhile, support is not quite at VMWare levels. Obviously, post-Broadcom, VMWare support took a nose dive, but Proxmox does not offer direct phone support, instead depending on resellers to do so.
Beyond that, I personally found the UI to be rather unintuitive, for example storage is defined at a 'datacenter' level, rather than at a 'host' level. PCIe Passthrough can be a bit...special, compared to VMWare having that be a trivial matter. Also, more sophisticated networking configs are more "Linux-y" in Proxmox than in VMWare, which uses a more traditional switches-and-ports paradigm that's much easier to understand and visualize. I also found VMWare's storage to be simpler, in that a datastore can have thin-provisioned VMs, thick-provisioned VMs, and installer ISOs all sit next to each other in harmony, while Proxmox gets more...particular. For example, an LVM-Thin volume will thin provision any VM stored to it and won't allow QCOW2 virtual disks to be added to it. ZFS storage is more flexible on that front, but it handles snapshots differently...
I say all of this as someone who either has moved, or will move, all of my VMWare clients to Proxmox. The feature set is more than enough for all of them, and as an added bonus, Proxmox is a *lot* less picky about hardware; I've got a box full of 10GbE NICs that got a new lease on life because VMWare decided the PCIe 3.0 Qlogic cards were 'too old', while Proxmox will still send traffic over a 10-Base-2 BNC network through an ISA card if I gave it one.