ZFS is amazing, but you do need to tweak the system so ZFS doesn’t starve your VM’s or host of RAM.
Allocate 1GB RAM per 1TB storage and you be fine.
Limit ZFS Memory Usage
https://pve.proxmox.com/wiki/ZFS_on_Linux
The problem with ZFS is, as per default it will take up to 50% of RAM available which is why many people have problems because they don’t tune it.