I'll preface this by saying I haven't even used Publisher yet. Just downloaded and am excited to start using based on my experience with Photo and Designer. With that said I don't see this listed as an export option and I think it could really help separate you from Adobe products. Export to the .mobi extension so that we can publish ebooks to Amazon. It would also behoove you to create an ePub export as well. The future is interactive design, I hope Affinity sees this and adds interactive export, ePub and .mobi support (especially .mobi as there is no good way to design ebooks for sale on Amazon currently).
Thanks!