In particular, scp is mindblowingly slow on lots of small files. I independently rediscovered the tar-pipe trick while sitting there watching scp laboriously copy thousands of 100-bytes so slowly I could count the files as they went by. That should not be possible, even at modem speeds. Fine for moving one file, OK for directories of very large files, not suitable for general usage where you might encounter a significant number of smaller files.
Absolutely. Connection latency hits you the hardest, since each file is sent serially, and requires 2 (or 3 with -p) round trips in the protocol, and this is on top of an ssh tunnel with it's own overhead. I can't remember what my tests showed, but I have this inkling feeling that tar over ssh was far faster than rsync for an initial load, since there's no round trips required, but you lose some of possible rsync benefits, like resume-ability and checksums.
If my first tar attempt fails for some reason, but it made a lot of progress, I switch to rsync. Best of both worlds. This hasn't come up often enough for me to script it.
