Why do you think like this? Because of the screen size? I think that I could definitely make a brochure on 12.9 iPad Pro. Obviously external screen support could help, but it is still very doable. Like LumaFusion, many people who edit their videos, use external monitors with their computers, but still, many edits on iPad Pro in LumaFusion as well. So, I think this could be still very possible. Even tho I use external monitors at home, in the office I use my 15" Macbook Pro and I have no problem to do brochures and such.
Affinity had said that they are working on the iPad version of the publisher, I hope that it comes soon.
Yes, I like to have a big canvas at hand when laying out and to see text flow and elements at multiple sizes and magnification levels. Sure it’s all doable with a tiny screen but that’s just not desirable for me.