Rectified edges, 1mm joints, uneven substrates, bowed tiles, thin tiles, unequal sized tile dimensions (out of square - not all tiles the same size in the same box/range) adhesive, customer/client need for perfect joint lines - all can produce problems for lip free/joint space tiling.
leveling systems help to stop lips - I, for one, want the clips to be called lip free clips and not leveling clips.
Many moons ago, tiling was done on a thick bed of sand and cement and that meant you could create your own substrate so that your tiles were fixed lip free........also tiles were much smaller and didn't have too much bowing although they were very ambiguous with sizing.