I learned how to use nested symbols in less than a day. It’s pretty easy. Sure, it may not be that discoverable to someone who is new to the sketch file but I find that a video demo or a walk through meeting can help with that.
Design is subjective. I remember in design class there were 10 principles of design that nobody could argue. #1 was legibility. Can’t remember the rest.. To me this list is like the ingredient list but it wonder if there is ever a top 10 principles that defines good visual design.