I remember that in the early days of WP Ultimo 2.0 development, Arindo and I worked on full S3 support for log files, invoices and other files saved by WP Ultimo.
With one of the last releaseses, that support suddenly broke and the files are not saved/readable anymore.
From what I see e.g. the log files saving functions do not use WP default uploads handling. I assume that this is the reason why it is not compatible.