Not every OS or every SSH daemon support byte ranges but most up to date Linux systems and OpenSSH absolutely support it. One should not assume this exists on legacy systems and daemons.
I agree but there are legacy daemons that do not follow the spec. Most here will never see them in their lifetime but I had to deal with it in the financial world. People would be amazed and terrified at all the old non-standard crap that their payroll data is flying across. They just ignore the range and send the entire file. I am happy to not have to deal with that any more.
I use lftp a lot because of it's better UI compared to sftp. However, for large files, even with scp I can pin GigE with an old Xeon-D system acting as a server.
I often find myself needing to move a single large file rather than many smaller ones but TCP overhead and latency will always keep speeds down.