I suspect this may be a difference of what we consider 'acceptable'. My results are consistent, repeatable, and match the experiences reported on this site and all over the web. If you are so happy with it why aren't you in those threads telling everyone reporting issues that it's fine? This doesn't even have anything to do with legacy programs - Microsoft's own programs and built-in elements all display the same behavior - it's not a matter of certain elements being the wrong size - it's a matter of the entire window being rendered fuzzy because it is being rendered for one resolution and then re-scaled for another depending on which screen was set as the 'primary' one at log in.
This is the behavior I see, and as I say, this is the same behavior reported all over the place. If you see something wrong, please let me know because I can replicate this behavior every time.
I set the Surface Pro to its native resolution and set the scaling slider to the middle (recommended) position.
If I am docked when I log in, it uses the attached monitors as the master size, so content is appropriately sized for the monitors and crisp. The menu bar is a little bit small on the Surface. Windowed elements are an appropriate size for that smaller screen, but many are somewhat fuzzy. When I undock, some elements like the menu bar remains small and the windows that were fuzzy remain so. Logging off then back on fixes this. Until I dock again.
If I log in undocked, and then dock, I get the opposite problem. The menu bar on the Surface is tiny. It is oddly sized on the external monitors with the program icons being smaller than usual for the size of the bar and the task bar icons being larger than usual. Still livable. Again, many of my windows are fuzzy - especially in Office and Chrome. It's subtle, but it's definitely not something I would want to look at all day every day. Sometimes elements are comically large - like the Control Panel. Other times they are normal sized but fuzzy.
Using the fixed custom resolution method works better because it makes the behavior consistent. Elements are still somewhat less than perfectly crisp on the Surface because you are working at its non-native resolution, but it seems to be a scaling factor that works better than what happens in the above scenario because it's still usable. I'll also always take scaling on the high DPI screen because all those pixels hide the issue a lot better than low dpi standard monitors. This is why things would have worked so much better with a 2880x1920 screen where you would have had the option of a direct 4:1 scaling factor which means everything stays crisp and extremely sharp.