It is always the unreadable that occurs. – Oscar Wilde
Typically the human brain can store only so much in its work memory at the same time. Too much stuff on the canvas distracts. Or worse, it becomes confusing or you might even get instant map shock. And, the confused mind says: NO. And NO means failure to get your message across.
So, try to keep the amount of stuff (objects used on the canvas) and the structure (the way boxes and lines are positioned, use of specific colors, text-boxes, and images) readable. 6-8 boxes per diagram are digestible, 12 already a bit more daunting, and 24 just plain crazy. Be also aware of adding too much meaning to certain things: The red line means A, the red dotted line means B, the bold dotted blue line means C, the solid yellow box and the black dotted line means D… well you get the idea. When your audience needs a guide in order to read your UPN diagram, you missed the point of UPN a little. Remember: If you lose your audience, you lose what you want to achieve…
And guess what: The hierarchical drill-down principle is there for a reason. Decompose complex steps into less complex steps.
There are situations when it’s ok to dump a plethora of activity boxes, sticky notes, and what have you, on the canvas: When you are not familiar (yet) with a process, or better, together in a workshop capturing process, you can easily end up with a lot of stuff on the canvas.
The skill to achieve, is the ability to categorize these (e.g., group related activities together) and then use the send-to-child function to put them 1 level deeper, creating less complexity. It is a skill though as you both need to understand the logic and need subject matter expertise.