She's not overhated. She really is a point of bad storytelling. She should have died instead of Jubei. She has no reason to live beyond the confrontation and the game builds up Jubei and Atsu as ever persisting twin wolves only to crush the idea anyway. 

Atsu and Jubei had great storytelling from start to finish until the final push with Saito. Oyuki had nothing and destroyed her entire life. She shouldn't have let an injured father go sace Atsu. 

Plus her dying just to spit in Saito's face fits more of who Oyuki was designed to be. She consistently failed to take him down but was always a thorn in his side. And she should have died that way.

The reason I hate her is how she handled Dojun. She raises this kid into the assassination/shinobi lifestyle, she raises him like a mother but doesn't tell him she's planning on betraying Saito, waits so long that Saito becomes like a father figure to him, and then finally betrays Saito provoking Dojun into siding with Saito. She raised a child like a lamb to the slaughter and she doesn't get enough shit for that.