Well, when DVD burners first came out, what speed where they at? 1X or 2X.
Why? my guess that the drive manufacturers had to find ways to make them more reliable at higher speeds incrementally improving, and not lack of proper media, these can show up later.
Also, why some people say that 16X is the theoretical max speed? because the media spinning can break and shatter at higher speeds than 16X.
So, my guess is that the top burning speed of a drive is pushing the technological state-of-the-art for mass-produced consumer devices sold cheap under $80. (Pro equipement might be more reliable.) You might find a well made drive and super good media and take chances. Some cautious people will not burn full disc, leaving off the last mega bytes for reason that that area might be less durable, this is the area burnt at 16X on the less stable outer edge of the disc.
Now, burning movie backup is not where we are normally looking for reliability and longevity, there are millions of same product in the world, and being converted to analog for viewing, we might experience some glitch, not loss of "real" archived data. But some posters here spend 3-4 hours trying to get better quality of the video, then save 2 minutes on the burn.
As for media it is the same, manufacrueres need to make improvements to get to the higher certified speed. And you also have to have faith the there is absolute consistancy between batches of blanks and even discs on same spindle.
As for longevity? there was a thread going couple of weeks ago.