Safe writing is clear, easy to read, and conveys the message. Typically, it follows a simple structure like introduction, body, and conclusion, or situation, problem, and solution.
Ninja writing does what safe writing does, but is also more engaging and memorable. It reaches both the heart and the brain. It is fun, playful, surprising, and interesting. While Safe writing is explicit, Ninja writing is a bit more abstract and lets the reader connect the dots.
Safe writing is closer to prose and science; Ninja writing is more poetry and art. Safe writing is more appropriate in serious situations where clarity and efficiency are paramount, like in a medical report or a supervisor's instructions. In most other situations, Ninja writing works better.
Someone wisely said, "If you try Ninja moves when you are not a Ninja, you may chop off your own head." That is a good example of Ninja writing. I remember that after many years and it brings a smile each time. The safe version of this is "Ninja writing is more powerful than safe writing, but it is also riskier and requires more advanced skills."
Safe writing is powered by clear thinking, whereas Ninja writing also needs creativity. My approach is to master Safe writing but to also keep dabbling in Ninja writing to build those creativity muscles, and to eventually fully graduate to Ninja writing.
Safe and Ninja styles don't apply just to writing. It applies to speaking, product development, sports, or nearly any skill. The highest level of any skill or craft is art.