Regardless of the number of drives available, you gain an advantage when your file system can leverage snapshots to roll backwards or forwards. There are other Linux-native filesystems that can provide this capability too, but many admins prefer ZFS because the full range of capabilities is unparelleled.
Perhaps I'm missing your point, but proxmox+lxc on zfs storage works fine in proxmox? If just looks like any other storage in proxmox and on commandline you've got all the usual zfs tools
I think it comes down to the standard argument against ZFS on linux -- uncertainty. It works *now*. Will it continue to work? Will any upstream changes in the Linux kernel cause issues with the ZFS modules bolted on top?
It is unlikely for there to be issues with ZFS and Linux. It's too common now, but it's not included in the main Linux tree, so it's not explicitly tested.
So, it's a low risk, but not zero risk.
More to the point here, when working with FreeBSD, ZFS is a first-class citizen (moreso even), so working with it *should* be more integrated with a FreeBSD solution than Proxmox, but how much more (and is that meaningful) is probably a qualitative feel than quantitative fact.