They are light but not as stable as others.
This means that they control less the bone segments.
This can bring to deformities more easy than with a stronger frame.
If you have a must for hiding your frames maybe they are a good option, but you have to run the risk of developing a deformity.
Hybrid frames (with pins and little wires) are the most stable thought a bit larger.
They can also be hidden under some larger trousers.