Classical unsolved control problems

In the book on Unsolved Problems in Mathematical Systems and Control Theory by Vincent D. Blondel & Alexandre Megretski it is written by the authors J.C.Martinez Garcia, M. Malabre and V.Kucera about partial disturbance decoupling with stability that disturbance rejection, disturbance attenuation are not the only methods to obtain internal stability and these methods work under rigid conditions and practically not possible to be achieved.