A cashier's check is equivalent to cash without the hassle, because it's backed by the bank that wrote it. They already took the money out of the customer's account, so there's no chance of it bouncing.
Similar situation with a money order. The cash has already been paid up-front, before the money order can even be printed. It's pretty much guaranteed money.
If the cashiers check is legit. A counterfeit one on the other hand...
Of course. But unlike potential counterfeit cash, all you have to do is call the bank to verify that the cashier's check is legit.
Some might consider having to Google the phone number for the bank, calling the bank, navigate through the bank's phone system, getting put on hold, and reading a bunch of numbers to the bank's employee a hassle.