The more parts of a plan that depend upon things beyond your control, the lower the likelihood of success.
You can't control people, or the weather, or institutions, or market fluctuations, or anything else that can throw a monkey wrench in your plan. It's risky enough having one uncontrollable thing in a plan, but sometimes our plans are built on several of them. The likelihood of failure increases exponentially. You could be doomed before you start. Unless you have a huge pile of luck. But:
The more complex a plan, the lower the likelihood of success.
The more rigid the plan, the lower the likelihood of success.
Flexibility is a virtue. Our bodies, for example, are rigid parts held together by soft stuff. The more flexy bits a plan has, the better. Because we can bend the plan to fit the situation instead of trying to bend things that are beyond our control to fit the plan.
I had planned on writing more, but I'm going to be flexible and end it here.