Since this is three short works, I’ll tackle this individually.
Lady Susan: While it’s different to have a novel mostly told through the perspective of a villain—I hesitate to use antagonist—this is really over-the-top. Lady Susan pettily laughs, her daughter sobs every five seconds, and everyone else goes “Oh no! What a horrid woman!” It’s like if Catherine from Northanger Abbey tried writing a novel, with the wild characterization and the way the book ends.
The Watsons: The problem with this and Sandition is that the text just stops with little or none explanation or idea of where the plot was going. The Watsons starts off as atypical Austen—young lady in society, is courted by wealthy and handsome gentlemen, but the fact that the book stops right as the plot is really beginning makes it hard to connect with. The editor’s note of Austen’s plans for the book doesn’t reveal much, either, as there are only vague plot details.
Sandition: Much like The Watsons, the plot stops as it’s getting started, but we’re not left with a short description of what could have happened. While these give some different styles Jane Austen worked on during her life, I’m really not a fan of unfinished products, if only because it’s so jarring to have the book end and knowing that you can’t really recreate the rest.