And there are also adaptable people that make it work under most or any system, and another type of people who find the failure cases for almost any type of system.
Both your examples mention single digits hours, ie very small amount of money.
The problems tend to happen if there's larger amounts of money involved. So it's important as a contractor not to enable a situation where they owe you a lot of money, especially if you have no leverage.
Yup, but make sure they can sell the product, not sell you.
Also, never give up control of the finances - abdicating that responsibility means you will probably get screwed. You need to be technical about ensuring your efforts are not stolen.