I work on Surface Pro 6, One main issue I am experiencing at the moment is that if I start using the surface pen to paint or do any action, then my mouse click isn't working anymore in the interface - I can click but nothing happens. At that point then I have to do everything with either the pen either my fingers. This is really annoying. This is valid for Designer & Photo.
Minor other issue is the scale when working with an extra monitor. If the second monitor is set to main and Affinity is used there, then the UI when opening documents, or the right click options are super huge. Other, if the surface screen is set to main, the UI for opening or saving documents is extra small.