This wasn't something I tested too much, how many 5GB+ test uploads do you want to do when you have a 250GB limit and regularly exceed it? I was always in my overage zone so that was costing me $1/GB. I wasn't keen to spend $50 troubleshooting this issue.
The upload would start well and it would use my full bandwidth, which at that time was 1.25MBps (or 10Mbps) until I got to about the 5GB spot. Then the connection would drop, it would re-establish, start uploading a bit, it would drop, etc. After a while Filezilla would just give up. I would try to restart it but I would see the same thing. But I waited a few hours and thena fter midnight I was able to start the connection and it went fine. This happened at least twice.
Is it a coincidence? Maybe, but I doubt it. I haven't tried to duplicate the issue recently since there are so many issues with the CGN3 to deal with.
But given my faster upload speed and higher cap on the 250/20 service I might subscribe to something like Backblaze to do cloud backups of my PCs. If that is the case then I will be sending HUGE amounts of data up to the cloud and will really give this a test, but I don't know if these services use FTP or other protocols.