Why do say they don't produce good PCB's? Conformal coating introduces problems too and, moreover, makes repair intractable. I don't know if conformal coating

Why do say they don't produce good PCB's? Conformal coating introduces problems too and, moreover, makes repair intractable. I don't know if conformal coating even works properly with the very fine pitch BGA's that cover much of the surface area of the PCB in an iPhone.