What the rocket scientist that came up with the idea for a magnetic pistol never took into consideration was the variations of the magnetic fields around the world, at various latitudes and longitudes. Also, a torpedo is not the most stable platform to be sending through the water, and expecting reliable depth-keeping to begin with, and then you throw in some stronger wave activity, and the nature of the pistol will detonate the torpedo, besides the fact that they could also trigger while arming, which had the potential to damage the shooting submarine. The rougher the weather, the higher the chance of a premature detonation.
The BdU was a lot like the US BuOrd, in that it was NOT their fault that a skipper couldn't do the trig to properly shoot a torpedo. "Nope, nothing wrong with the weapon." At least they came to realize the problems with the device before the US did... Some US skippers would counter-mand their orders, and then ran the risk of being written-up upon return to base. In the game, those messages are "immersion" treats for the player, but there is no penalty possible in the game if you do not follow orders. If you want to be authentic though, you follow orders...
There are quite a few articles on the subject. As an example:
https://uboat.net/history/torpedo_crisis.htm