In my personal experience I have bad memories of Steam Preload, for Monster Hunter, The Witcher 3, and a couple more games, Steam didn't preload correctly and mostly downloaded packed archives to unpack, so I downloaded the files only to unpackthem later, which took double the time if I'd only download the game when it was available.
I understand some people still find this usefull because they have data caps, terrible download speeds, but that's not my case, I can dl 50gb in about 10 minutes, while the unpacking and stuff would take around 15-20 minutes on my computer.
For me, Steam Preload is useless.
I would love to see the EGS doing a more effective pre-load system in which you download all the game but the .exe, and when the game is released, you just patch it with the .exe and you are set. (I don't know shit about computers)