Why is using bond enthalpies the least accurate method of finding reaction enthalpies?

[John Huang 1G]

I remember Lavelle discussing this, but I can't seem to recall why this is the case.

[Cooper1C]

Bond enthalpies are averages of many bonds.

[Harsha 1J]

And I think the bond enthalpies are values under standard conditions, which might not always be the case.

[Sarkis Sislyan 1D]

Only diatomic molecules have accurate bond enthalpy measurements, all other molecules' bond enthalpies are taken as averages

[William Xu Dis 1D]

While formation enthalpies are very precise, bond enthalpies are calculated as the average of many bonds. Even if you take the bond C-H, there are many different ways it can be formed into in different molecules.

[Sarah Sharma 2J]

Bond enthalpies are composed of multiple averages and therefore are an overall average. Whereas other values nad methods such as enthalpy of formation are much more accurate.