I often use "user waypoints" fixed wing.
To avoid airspace, or just to simplify a visual approach to somewhere, I often fly a radial towards one VOR/DME, and then at a certain distance from it I turn onto the next leg. Situational awareness can only be helped if that turning point is also on the GPS moving map, but it won't be in the database.
So long as you cross-check the user waypoints (plotting them and checking visually is best, pre-flight) I don't see this as any more dangerous than entering the wrong "Bangor", or even mistaking one town for another on a chart!
To err is human, that's why we cross-check.