Don't think its getting easier. I believe there is more than one person setting these puzzles as it seems easier every 2-3 weeks. Also when its not 'easy' its because there is a proliferation of foreign/latin/ancient words. A lazy way to construct the puzzle.
the concise one in the sunday times is often quite difficult and i do wish they wouldnt use one word instead of the correct two, , its snowing again so am feeling fraught