Neither the Born Lande or the Kapustinskii Equations Correctly Calculate

Neither the Born Lande or the Kapustinskii equations correctly calculate the lattice energy for beryllium compounds. Why?

A) The Born Lande and Kapustinskii models are based on ionic bonding and the bonding in beryllium compounds is largely covalent.
B) The ionic radius of beryllium is an estimate as no beryllium compounds have been isolated.
C) Beryllium compounds do not form the usual lattice types and no value of the Madelung constant for their structures have been calculated.
D) The first ionisation potential of beryllium is not known.