I am often in a situation where I need to take notes on a computing device while having it in my lap for work. I have tried various iPads in this configuration - they are all inferior to laptops for lap typing for this purpose. I am guessing the 12.9” magic keyboard will be better than a 12.9” with a Smart Keyboard folio in the lap - however both will not be as good as a laptop on the lap.
I think a lot of reviewers have made this point - the magic keyboard, and trackpad support in general, have made the iPad Pro (particularly a 12.9”) a monster desktop machine. I am sitting here with my 12.9” on a table with the old smart keyboard folio and the magic trackpad 2. I now use the Magic Trackpad to navigate IOS, and it is comfortable as I can lean back in my chair and control the OS without touching the screen. When I want to use it in bed, I am now folding the keyboard back or taking it out of the folio entirely. But on the desk, I think the Magic Keyboard (plus the Magic Trackpad 2 - I intend to keep using it with the iPad) transforms the iPad into a different more powerful machine