What is the disadvantage of Printing shield can on the plastic?

Printing shield can on the plastic seems more cheaper than others.
But what is the disadvantage of that?
What is the shieding effect of it?
Should the plastic be absorb materials?
Any comments are welcome.

This brief has some info on what plastics can be plated and how (electroless, electrolytic). Also, some numbers for shielding effectiveness.

Hi, pstuckey,

thank you very much.