safety of extremes

The great artists and the bad artists are easy - it is the good artists that can kill you. With the great artists you just keep putting fuel in their tank. With the bad artists, you realise your mistake quickly and cut your losses. It is the good artists that bankrupt you because they are good enough to make you think they are about to turn the corner and therefore keep you spending.

Blockbusters - Anita Elberse (Page 81)

Similarly, you do not have to be afraid of stupid or smart people. You will feel tempted to set up mechanisms to keep the stupid ones in control and you will confident delegating responsibilities to smart ones without any worries.

Those in the middle will constantly fool you and turn out to be the costliest.