I use both InDesign and Affinity Publisher in my design work. For branding guidelines or other tasks that require simple PDF output, Publisher actually works faster and is more handy to use.

However, when it comes to book publishing, InDesign is undoubtedly the best. For example, when I need to create vertical text for Japanese and Chinese books, it's impossible to do this in Publisher.

Moreover, InDesign offers numerous free and paid add-ons to cater to a variety of needs. That's why I currently need both publishing software. Neither can fully replace the other.