Any downside to feeding a buck regulator into a linear one?
The only drawback is loss of efficiency if any. Depends on how efficient a 12 to 3.3V linear regulator would be or a 5v to 3.3v switching regulator would be. Otherwise, a linear regulator typically tends to smooth and is less noisy than a switching reg, and has less complicated layout requirements. Everything is a trade off, but 5V to 3.3V is often done with ldo instead of switching regs for noise abatement reasons