Not the GP, but as an ESXi user considering converting to Proxmox, there are a number of limitations on the free ESXi that can be annoying, largely around the non-standard and generally poorly supported tooling required to manage the platform unless you have a paid vCentre instance.
I can't easily create new VMs, create snapshotted/linked clones, and I can't easily pull out CPU/RAM/disk utilisation information from ESXi for the hypervisor and all VMs to store in a time series database for monitoring/alerting purposes.
There's also a tonne of additional features in Proxmox that requires additional licensing for ESXi (and yes, I am aware of the VMUG EE licensing making a lot of that a lot more affordable)
Proxmox ultimately being Debian based gives a lot more flexibility in that regard. It's also a potential weakness as a result, depending on perspective.
