Have you checked out duplicacy[1]? It's support for incremental backups is fairly unparalleled in my experience. It does not have the same limitation of duplicity.
I know what you mean. It is not a normal open source license and for commercial use it requires a $20 a year subscription per user. With that said, I believe all the core source code is viewable and auditable if you use the CLI version, which is what I use on my FreeNas system and there is no charge for personal use. I think this may meet many people's personal backup requirements.