If you are often restarting rsync jobs or they fail over low speed WAN connections

    --partial               keep partially transferred files

Is your friend. Esp if you are trying to sync directories with multi GB files. Also if your media is bad or there is a failing piece of network gear along the path

    --bwlimit=KBPS          limit I/O bandwidth; KBytes per second

Can keep things running smoothly. It even works against disks, I have used it where an array would die if one attempted to read from it too quickly.