I've hardly ever used the Sketch feature--mainly because it's not easy to do on a phone. But I just tried it on my new iPad Mini 6 and have thought of several good uses for it. A finger works quite well when I'm sitting at my desk at home. In turbulence, however, it might not be so great. But I also tried writing and drawing with a stylus--not an Apple pencil (which I don't have) but a cheap passive gizmo, like a short pencil, with a soft, rubbery tip. It's easier to control than my finger. But it's also a lot easier to lose, so I probably wouldn't depend on it in flight.
Now I have a question for you: Does a sketch drawn on a map survive map updates? I'm thinking of Sketching wires (power lines) across rivers and lakes that aren't charted. Wires are death to seaplane pilots because you can almost never see them, and they're rarely charted. I've been setting a user waypoint when I come across a wire over water. That's effective; but it's hard to settle on a naming system for them. "Wire 27," for example, works, but I never remember what "wire" number I'm up to. So it might be better just to draw a wire right on the Map instead--IF it becomes permanent.