There are limitations to using mean bond enthalpies to calculate enthalpy changes.The actual energy involved in breaking and forming the individual bonds will be different from the average bond enthalpies given in the data book, since these have been calculated in different chemical environments.All species need to be gaseous molecules.Using thermochemical cycles to calculate enthalpy changes gives more accurate results because thermochemical cycles use data specific to the compound whereas mean bond enthalpies are averages and are less specific and less accurate.
Mean bond enthalpies allow you to calculate enthalpies for compounds that have never been made.