SGU was awful IMO. They tried to make SG DS9/BSG and young adult fanfic. The whole thing was tonally the wrong direction. Atlantis was OK, but it definitely started the trend in that direction. There were darker elements and heavier themes in SG1, but there was always a bit more levity and grace.
It's the same thing that happened after TNG. The difference is the outlook of the world building generally. In TNG and SG1, it was generally dynamic and upbeat, you see that shift when the crisis happens in the episode. You want to live there during the best glimpses because it's a positive setting. In the later shows, everything became dark and depressing. All of a sudden the federation is corrupt and doing weird bullshit, the various entities in SG1 become fixtures of the settings (like Woolsey) and SGU is visually just not very interesting.
