Yeah I almost never use splines on a surface in SolidWorks, or the 3D sketches at all really. I just recently got a chance to try it out in Creo and it is much better. If I do need to have a curve on a surface I’ll usually project a 2D curve onto the surface. It the example you showed I’d probably make a plane that gave me the best view of the sketch, usually a plane roughly tangent to the surface in the middle of the shape you’re creating (or offset from that) or possible just the most common/important view. That way I have a decent sense of the final 3D shape in a 2D view. If I am using a spline on a surface I’ll use constraints on almost all the handles, whether it’s to existing geometry of a projected curve like cwatkinson mentioned. If I don’t have a good constraint for a handle I like to leave the points without handles since it’s really hard to manually adjust them, especially since you can adjust them in any direction.