The Elastic Modulus (rigidity) of fiber posts is about the same as dentin (18~20GPa). 

Titanium is 5 times as rigid, Stainless Steel post is 10 times more rigid. 

This mechanical mis-match predisposes to damage to the tooth including root fracture.

Most fiber posts are tooth colored or translucent, therefore aesthetic.

Fiber posts cannot create a galvanic or corrosion potential, as stainless steel can.

Fiber posts are removed quickly by drilling through them, rather than by removing surrounding tooth structure.