If you see the dirty blob, here are some things to consider 1. Switching between box and brush, in most cases one of them would work better. 2. Box would be preferred for full object removal, Brush would be good for texture retouch, but it could vary some times so try both. 3. The result won't be pretty on big object removal. 4. Remove the dirty blob or "Undo" and box/brush again.

We are working on more advanced backend to get a cleaner job done, sorry for the frustration. Thanks for trying it out and the valuable feedback!