To understand the dilemma of molecular geometry we need to recall the shapes of the atomic orbitals.

With such orbitals the bond angles in molecules should be pretty much restricted to multiples of 90^o. It is difficult to see where the 120^o and 109.5^o angles would come from.

Also, some covalent bonding is nearly impossible to explain based on electron configurations. If a typical scenario is for atoms to share unpaired electrons to form a bond, some very ordinary molecules would be out of luck.

Consider a simple molecule, BeH₂.