Post crisis reflections on Central Bank Reserve Management

"Mechanistic reliance on credit ratings can lead to forced selling in stressed market conditions (thus making the stabilization task of other central banks more difficult) and unintentionally increase concentration risk in the remainder of the portfolio".

http://www.imf.org/en/Publications/WP/Issues/2018/02/16/Central-Bank-Reserve-Management-and-International-Financial-Stability-Some-Post-Crisis-45635