Ejection seats have a ‘window’ where it is considered ‘safe’ to eject. If you are outside of that ‘window’ your odds of surviving the ejection are slim.
Here is a first hand detailed description from a pilot that beat the odds and lived to tell the story of ejecting at supersonic speeds.
Featured image of Capt. Matt Bruckner, an F-15 Eagle pilot assigned to the 71st Fighter Squadron, 1st Fighter Wing, at Langley Air Force Base, Va., flying a combat air patrol mission 7 October 2007 over Washington, D.C., in support of Operation Noble Eagle by U.S. Air Force photo/Staff Sgt. Samuel Rogers