I believe the assured rate is the rate that they would take any action with their wholesale provider if the actual throughput is lower (than the assured rate).
I may still get what I get now on Duo Advanced, but Uno are going to be in a significantly worse position in terms of escalating to their wholesale network provider if there are performance issues.
For me, it comes down to a question about whether I potentially want to risk compromising my broadband connection to save money (and the saving is not inconsiderable - £14 per month compared to the monthly equivalent of my annual fee with IDNet).
I get the feeling that this is a product designed to compete with the mass market products and may be found wanting by more discerning users (e.g. typical TBB forumites), but that is just conjecture.