On board or Off board, you still have to be cautious.
Exemple 1: years ago, it was a ZeroUAV card with external compass-GPS
I was not able to calibrate, guess what? I had the GPS puck over a 3mm steel screw…
Exemple 2: an equivalent PX4 card, mag on board, on the frame with Battery cable loop below, bad calibration results as expected, without loop, it is better.
Exemple 3: a modern card with no compass on board, and 2 compass on the GPS puck, no problems if you are cautious with GPS placement.
Exemple 4: …
There are as many situations as users. We have to be cautious with our installation and settings. Good results can be obtained with moderate cost hardware and bad results with hight end products. Clever action should produce good result.