So basically the simplified surface you want to construct should look similar to a big rectangular sheet. Then the final more detailed non-rectangular boundary is formed by cutting that surface either with Trim or by booleans.

If you try to construct the initial surface directly to an irregular boundary the shape of the boundary is going to have too much influence on the surface shaping. The surface will be stressed and be pinched or stretched awkwardly and won't have good shaping.

- Michael