The simulatable stuff is almost perfect. It's the stuff that can't be simulated that fails.

Take the last flight as an example. The booster experienced what was (probably) a structural failure in the propellant fuel lines. Simulating stress in the structure under static conditions is quite straightforward. Simulating the stress as the rocket ascends vertically and the tanks empty is hard, but doable.

Simulating the dynamic loading as the rocket flips? The fuel sloshes around, the sloshing fuel changes the kenimatics of the rocket, the kenimatics of the rocket change how the fuel sloshes, the engines try to correct adding a new force, the thrust from the engines creates increased force on the fuel increasing the pressure to the pumps, the performance of the engines changes because of the new fuel flow, that alters the acceleration further causing fuel to slosh, gass bubbles are intrained in the fuel from all the sloshing thus altering its flow/sloshing behavior, valves open and close creating pressure waves in the fuel that travel up and down the fuel lines (the water-hammer effect alone being enough to burst the pipes if valve closing is not well-timed), and the rocket itself flexes as all this happens, testing every exact detail of the manufacturing which you have to go out to the factory and physically measure. No simulation software ever imagined can handle all that coupling of systems.

The usual solution is to make some conservative estimates (the center-of-mass of the fuel will move by at most some amount, bubbles will last at most some time, the engines will have so much control authority, etc). But that requires experience. And this is aerospace, so safety margins are tiny.