there's no such thing as a 100% safe base.
you can have 20 fusion defenses and 5 grav shields, and statistically speaking, there's still a probability (however immeasurably small) that the aliens will make it through, exactly like in the original.
think of it this way: in a complete set of possible outcomes of 100 random numbers from 0 to 100, there exists one outcome that is all zeroes.