Using magnets will change the output of the pickup, and only the pole pieces which the new magnets are attached to will be affected giving an uneven output across the strings. You can try this by adding a flat magnet across the underside of a pickup to increase output.
Also, although the magnets will be fairly strong against a pulling apart action, they'll be fairly weak against the shearing force of resting your thumb on the ramp edge, if you do.