Anything human made is prone to all the errors humans make. At least with Open Source, there are more eyes that can spot mistakes, potentially even provide a fix.

Sure, that means bad actors can find them too. But closed source doesn’t prevent that: Raising the hurdles may slow them down, but they if they have a financial incentive to keep trying, it won’t stop them as effectively as it stops the type of people who would do a responsible disclosure instead of selling the information.