Ask HN: What are the challenges facing LIDAR today?

So basically, my question is what the challenges are to high resolution automotive LIDAR like the Velodyne becoming cheap and mainstream. For example, I hear that solid state LIDAR is supposed to be the next big thing so we can replace the mechanical scanners. What are the challenges holding back solid state LIDAR? Ideally information for all of the three major versions( MEMS versions, flash LIDAR and phased array versions), but any information at all is useful!