In terms of size, the Surface Go is in between the 9.7 and 10.5 iPads (but much thicker - same thickness as current Surface Pro). The Go is perfect for people who want bigger bezels - they’re bigger than the bezels on the SP4.
Maybe YOU can’t be productive on an 8” display, but I certainly can. Not only the iPad mini, but I also had a Dell Venue 8 Pro and it was great from a size perspective! I had a great setup where I used the Venue 8 with a Microsoft foldable keyboard and a ArcTouch mouse (as well as a Stylus). I had a couple trips I took this and my iPad along and it worked really well. I stopped using it because it finally became too slow.
If the Surface Go was smaller (MUCH smaller bezels, reduce the width/height - maybe even an 8” screen) it would have been an instant pre-order. As it stands, beyond size, there’s no big advantage over my aging Pro 3.