I use pg with zfs on freebsd,it work great.I test zol 1 years ago,it will crash the os on load.
Some note for pg on freebsd from my experience:
1.if you use compression,8k recordsize make the compression ratio poor.I reach 7.x with gzip using default record size(128k if I remember) while get 2.x with 8k record size.
2.use slog to reduce fragment,and improve the sync write performance.