I came across the enclosed reviews of Alien Covenant and thought the authors did an excellent job of presenting the themes Ridley Scott sets out to accomplish in this movie and in Prometheus:
http://www.comingsoon.net/horror/reviews/850735-the-majesty-and-horror-of-alien-covenant#/slide/1
https://thepopcornjunkie.com/2017/05/21/the-gods-and-monsters-of-alien-covenant/
Combining the the themes and story arc begun in Prometheus, followed by Alien Covenant which takes place 10 years later and 17 years before the events in Alien, these "prequels" combined dovetail nicely with the Alien and Aliens universe and "fill in a lot of blanks" = questions unanswered before now in the earlier films.
My biggest problem with Prometheus was that the “aliens” did not seem to be of the same species as the “A L I E N”s. It seemed like a completely different story, and it threw the whole thing off for me. I am given to understand that Covenant explains this. Not sure I’m willing to pay full price to find out.