rsync went quite fine while transferring data (in the same situation as you describe), when taken care of some important bottlenecks (not running it over SSH, disabling compression on files that don't compress well, disabling full checksums, TCP sockopts, ...)
what it might leave you hanging with for a long time is before an actual transfer, while it builds and compares the lists on both sending and receiving side, when you have big filesystems (hundreds of millions of files).
if you have a strategy to select beforehand which files to transfer (for example from a DB which tracks what has been created or changed, direct from worker or production input) you have a good headstart and can minimize rsync on complete filesystems -- and rather run it on a selection, which is tiny compared to the complete project(s) most of the time.
