Convinced that the folks on Planet 4 are not Engineers

My guess is that the Engineers are like the Mayans, or Aztecs, a kind of culture. One possibility is that there were other cities on that planet that were more advanced.

I didn't have a problem with how their buildings looked although they could have been better. Eventually people have a tendency to make up things in their minds and they get disappointed when the result doesn't live up to what they have imagined.

Agronut: I think that they over-did the David thing in Alien Covenant. The next movie needs better human characters because there were no one to root for in AC. They should have had The last supper and Phobos in the movie because it explained the characters better. It was a mistake that they didn't have those. No thanks to another David movie, I didn't like that in Covenant that much even though the movie was alright (2 out of 5 is what I give it).