I've thoroughly read through Advent Horizons now and have grown to appreciate its unique blend of 3rd and 5th edition on character generation. However, I have been unable to motivate myself to run it....and that can be a real issue for getting any game off the ground. Here's the two reasons why AH is as yet unplayed by my group, and what AH needs going in to the future to attract more players. I hope this happens, because I think it's the best iteration of D&D 5E in a science fiction context I've seen so far, but it still needs some serious fixes.
The AH core book includes a barebones approach to system generation, plot generation, a handful of foes and that's about it. There is not enough to get a GM properly up and running here....at least not one like me, with limited time to design a lot of content. The background provided is a good skeleton, but it could also use a sample area with more detail and maybe a starter scenario. Really though it needs an "AH DMG" tome to supplement the core rules.
The book needs a serious edit from a proper copy editor, not merely a word processor proof-read. I think the straw that broke my back on this was reading about an NPC's "quarks." I closed the book at that point and gave up.