Pre-Epilogue Sadie is a bad character. Her whole story is pretty generic, they didn't even try to do something new with the revenge story genre. I didn't like her, she did tons of dumbass things for no good reason which caused others to suffer (most obviously her causing Mr Bullard's death, which was a pretty brutal moment even though he was a minor character). But I also didn't hate her, as pretty much everyone in the gang has killed people or caused people to be killed, you could even say she just made a lot of mistakes due to her past trauma (which isn't an excuse but is an explanation). Because she's not a likeable person but also not an antagonist, it just makes her forgettable. Her psychotic outbursts against the O'Driscrolls were almost just painful to watch, mostly because her dialogue feels pretty forced. She is just unable to shut up about her husband, of course I wouldn't expect her to just get over it, but it is pretty annoying when she keeps mentioning it (especially in Goodbye, Dear Friend where she undermines Arthur getting captured and tortured with the logic "you survived and my husband didn't"). You don't see John constantly talking about Micah killing Arthur, it would've been just as annoying.
I didn't even particularly like her relationship with Arthur, it wasn't even on the same level as Arthur and John, which I guess is a high bar to cross but still.
After Chapter 6, though, when the O'Driscolls were gone and she stopped talking about her dead husband with bad dialogue, I liked her more. She stopped being whiny and was actually pretty badass at times.
Of course this is just my opinion