IDIOMS

devil is in the details

(idiomatic) The specific provisions of, or particular steps for implementing, a general plan, policy, or contract may be complicated, controversial, or unworkable.

Example: 1998, Alain L. Sanders, "Israel: Let the People Speak," Time, 16 Dec.,
  The country has developed a strong consensus for peace, but the devil is in the details over what concessions should be made.