Plastic frame eyeglasses have several disadvantages compared to other materials like metal or titanium:

Durability: Plastic frames are generally less durable than metal frames, especially when exposed to frequent bending or rough handling. They can break or crack more easily.

Allergic Reactions: Some people may be allergic to certain types of plastics used in frames, leading to skin irritation or allergic dermatitis.

Weight: Depending on the type of plastic used, frames can be heavier than metal frames, which may cause discomfort or leave marks on the nose and behind the ears.

Heat Sensitivity: Certain types of plastic frames can warp or lose their shape when exposed to high temperatures, such as leaving them in a hot car.

Adjustability: Plastic frames may be less adjustable compared to metal frames. They can sometimes become misshapen over time, making it difficult to maintain a proper fit.

Style Limitations: Plastic frames may offer fewer options in terms of style and color compared to metal frames, which can limit choices for fashion-conscious individuals.

Maintenance: Some types of plastic frames can be more difficult to clean and maintain. They may require special care to avoid scratches or damage.