There is no benefit to using f2fs on /system. The main benefits of f2fs are arguably less wear on the nand and write speed. /system is generally read-onlyRegarding f2fs:
I moved /cache and /data to f2fs but I couldn't move /system. When I restored my /system-backup onto a f2fs-formatted /system partition it wouldn't boot and flashing the installer on a f2fs-formatted /system-partition reflashed /system to ext4 before installing the zip.