Technically routing gets easier with more layers available. If a 16 layer board cost the same to manufacture as a 2 layer one I would just always use that. Of

Technically routing gets easier with more layers available. If a 16 layer board cost the same to manufacture as a 2 layer one I would just always use that. Of course today they are still magnitudes more expensive so generally such a high layer count means you are routing evil high frequency signals..