Fundamentally, Section 31 only works for me if either it isn't as black ops and evil as it made out to me, or if it is exactly as black ops and evil as it is made out to me, but shown to be highly problematic and counter-productive, something that needs to be stopped for the good of the Federation. Anything else is antithetical to the ideals of Star Trek.
I'd say it only works as the latter, because it's been well-established over multiple seasons that it simply factually is the latter, and that cannot be avoided. Trying to retcon it into like "NAUGHTY PEOPLE CLUB" (or, worse, "MISFITS" as the trailer suggests - I'm sorry but Georgiou is Pretty Privilege Pol Pot, she's not just "naughty" or a "misfit" lol - she was the opposite of a misfit in her society, too) is pathetic and awful.
Actual Star Fleet finding out about S31 but not being part of it should simply be there to STOP them. The entire concept of S31 is that they operate without oversight, without responsibility, and frequently
directly against the stated goals, values, and rules of the Federation.
They could have done something okay like, semi-reformed Georgiou joins S31, finds out they're completely cracked and going to do something totally insane that would be so bad even she thinks it's bad, and works to stop them from doing that whilst perhaps forging them into her own little Black Hand.
Instead this looks like a crummy version of Suicide Squad, as several others have observed. And not the kinda-decent The Suicide Squad either, the pre-The one.