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.