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They are the least accurate because they are an average of all the possible molecular structures they can be in. If you actually compare the methods they are actually pretty similar, but the other two covered methods deal with more exact numbers based on the specific structures in the equations.
Because of the way bonds work (dipole moments, random movement of shared electrons etc.) there is no single enthalpy for pretty much any non diatomic bond. bond enthalpies are calculated by taking the average of many different bonds of a particular kind.
Diatomic bond enthalpies tend to be accurate, however every other molecule/bond in a chemical reaction has bond enthalpies made up of an average of thousands of different measurements. This averaging makes it inaccurate.
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