If you measure useful energy as electricity output of a fossil fuel plant then yes. But in many cases the waste heat is used in other applications for example district heating or low grade industrial heat.
If you use fossil fuel to directly drive an industrial process, for example melting of ores/metals/glass then the efficiency is much higher.
If you use fossil fuel to directly drive an industrial process, for example melting of ores/metals/glass then the efficiency is much higher.