Uchikomi, drilling, with a partner, yourself, or with a dummy all suffer the risk of being unrealistic. But I think as long as you have an understanding of the technique, you can use all those methods to supplement your training.
Here’s a video of a dummy in a gi being thrown beautifully. This is a very exciting video for me, a person who hates it when I can’t find a throwing partner:
Here’s a less useful (for me) video of wrestlers throwing a dummy with drop seoi nage and koshi guruma:
And as long as we are going in the order of decreasing effectiveness, here’s a video of a guy being admirably creative, but really getting too far away from the real thing: