Wait a minute, is Amazon truly UNLIMITED? Even for encrypted files from ARQ? Mine currently is going to OneDrive, but that's limited to 1TB for $7 a month which includes office 365. There has not be some catch with Amazon here.
Personally I have close to 7TB of backups and personal video footage stored, everything encrypted, never had any issues.
However there's always the danger of them pulling a onedrive at some point in the future and reverting from unlimited back to something like 1TB if people actually use it.