Bond energy sums are demoted; ΔHf and Born–Haber take over

ΔH = (sum of bond energies broken) − (sum of bond energies made). Presented as the way to calculate energy changes.

That method survives (Reactivity 1.2.1) but is explicitly labelled approximate, because tabulated values are average bond enthalpies over many compounds, and because it is only valid for species in the gaseous state. The more accurate routes are Hess cycles from standard enthalpies of formation and combustion (1.2.3–1.2.4), and for ionic compounds the Born–Haber cycle (1.2.5) combining atomisation, ionisation energy, electron affinity and lattice enthalpy.

The student acquires a hierarchy of methods with known error characteristics and has to choose. IB questions explicitly ask "explain why the value calculated from bond enthalpies differs from the experimental value" — the answer being average values and state (bond enthalpies apply to gases; if a reactant or product is liquid, you've omitted the enthalpy of vaporisation).

Bond energy sums are a self-contained arithmetic exercise; ΔHf requires the idea of a reference state.

Applying bond enthalpies to reactions involving liquids or solids and being confused by the discrepancy; not knowing which sign to attach to each leg of a Hess cycle; treating lattice enthalpy sign conventions inconsistently (IB uses lattice dissociation enthalpy as positive — teacher should confirm the convention used in the current data booklet).