Original magnets remaining can still come loose at any time.

Magnet alignment:
The alignment of magnets around the flywheel is critical for proper stator operation. Even very fine misalignment will reduce stator efficiency significantly.

Magnet spacing:
Excessive epoxy or incorrect magnet spacing around the flywheel can easily affect the flywheel balance. This can cause stator contact or even crankshaft damages.

Incorrect replacement magnets:
The magnetic field strength of new magnets MUST match the originals. Stator failure can occur quickly with stronger magnets and stator output can be decreased significantly using weaker magnets.

Wrong epoxy properties:
Epoxy that is not rated to permanently handle high temperatures can quickly cause a serious failure.

Oil contamination:
Wrong epoxy for the application can also contaminate the crankcase oil, damaging the motor.