To take an extreme example, you could ensure that validation happens identically on both the back and front end by writing your own framework with that property. You could create a framework with no more security bugs than the next best alternative and while providing great UX. But writing a framework and shaking out the bugs is a huge lift.
So in practice you can't go all the way out on the third axis and it is approximately a dilemma. But if you're on the lookout for exceptions you may find an opportunity to cheat/curve bend (eg as suggested by other commenters, when using JS for the front and backend, you can use data modeling libraries like Zod to get most of the benefit for a fraction of the price of writing a framework).
