I find the deform tool produces stable, smooth results on an extruded form, but when the solid is assembled from surfaces, I'm not getting it to work.

At A in the image I have a bent solid that's made from a mix of flat and bowed boundary surfaces, made with the spline lines that are visible. They're knit into a solid and fillets added to the corners. This will become the body of an electric shaver, which will be comfortable to hold if tapered some in the middle. At B I've created the deform tool that attempts to taper in the center part.

The application of the tool at the smallest degree of strength, in the form of the smallest dimension SWx allows, leads to the erratic form. Deleting the fillets doesn't help.

If you picture this as something SWx performs and expect to work, what feature and option choices do you picture as working? I've attached the file if you're motivated to adjust some points.

Edu ​​​​​​​SOLIDWORKS ​​​​​​​