I've been seeing pretty bad upload failures (probably around 30%) for uploading hundreds of 30-40 MB files per month to B2 from New Zealand since I started using B2 over a year ago.

And I'm not convinced it's connectivity issues, as I can SCP/FTP the same files to servers in the UK...

When I test using an actual software client (Cyberduck) to do the same thing to B2, I see pretty much the same behaviour: retries are needed, and the total upload size (due to the retries) is generally ~20% larger than the size of the files.