Although 3S has less lively stages than other SFIII games, their art design is so on point it's hard for me to take seriously the opinion that they're in any way bad. Preferences are subjective and everything, and I also prefer the bgs in other SFIII games, but to use that as a reason as to how 3S's bgs are bad or on par with SFIV or V is baffling to me and shows an extremely, extremely bad artistic eye.
Are the bgs in 3S the best in the series? No. But they're not bad. In this case, "great" is the enemy of the good. It's understandable. More characters, aging hardware, and how damn animated the bgs were in past games that the stages would take a hit. But the aesthetic is on point and combined with the music, it's very pretty to look at and listen to. It's a nice looking game.
Also, personally, I like the desolate feel because I'm huge on atmosphere "lonely" SF stages as seen in SF Alpha games. Going by most 3S background haters arguments, Alpha 2 also has dog shit bgs. I find 3S stages just to be a continuation of them.
Same feel, different game.