I would imagine it would cause issues with the IMU depending on placement due to the calibration routine done in the factory, unique to each controller. There's also things about this covered in the HDK that I'm not sure if they are or are not covered under any NDAs I've signed, but suffice it to say that there might be some issues. As another user mentioned in response, slight variations in conductivity are at the heart of many sensors like hall effect sensor in the trigger, any type of inductive sensors, and a number of other sensors and scenarios, so potentially (though this is just conjecture) a powerful magnet, or any ferrous metal too close to the heart of the controller (fpga, mcu, soc, etc.) could cause issues. I'm fairly confident I've only covered things that were public knowledge already here, but there are additional design considerations that this goes against which would negatively impact the performance from a tracking standpoint and an electrical engineering one. I just don't believe I can go into detail on them, though anyone with access to the HDK now that it's been opened up to a wider audience, or even the Vive Tracker documentation can probably go into more detail for you and anyone else interested.