One of my ZFS servers used to have 4G of RAM. It had problems with kernel memory allocation so I upgraded it to 12G. It's the first time I've ever had such problems on a system with 4G and I really didn't expect it from such a light SMB and NFS load after I had made the recommended changes to limit the size of the ARC.
Yes, I have deduplication turned off.
I agree that integration with Linux is an issue. With BTRFS you have all filesystems listed in /etc/fstab while with ZFS they are all managed by ZFS software without a mention in /etc/fstab.
BTRFS is more like just another filesystem to use, while ZFS is something that totally owns your server.