Bleaching agents do not work on materials such as plastic or porcelain, which many dental restorations are made of. If you have such restorations, especially in the visible area of your smile, bleaching can lead to an uneven colour result where your natural teeth become lighter but the restoration remains unchanged.
If you have many defective fillings, bleaching may not be feasible. Defective fillings can cause the bleaching gel to penetrate areas it shouldn't reach, which can lead to uneven results or damage.