rclone is super cool, but unfortunately many of the providers it supports has such low ratelimits, that it's fairly difficult to use it to transfer much data at all.

This has been my problem. Not necessarily that the rate limits are low, many can be gotten around by using multiple users to do the work since the limits are per user, but how rclone handles those rate limits when they hit them. The exponential back off will end up making hours and days long delays that will screw a migration.