The “secret” to Stable Diffusion (and other CLIP-based models) is being as descriptive as possible. This prompt, whilst easy for humans to imagine, actually has a whole lot of ambiguity baked in.
How high is the unicorn flying? Is the unicorn even flying, or on the ground? How old is George Washington? What visual style is the image in? Is the image from the perspective of a pedestrian at ground level, or from up at skyscraper level?
The more ambiguous the prompt, the less cohesive the image.
To demonstrate, here’s 4 renders from your original prompt: https://imgur.com/a/Jo4qfOp
And here’s 4 using the prompt “George Washington riding a unicorn in Times Square, cinematic composition, concept art, digital illustration, detailed”: https://imgur.com/a/lB36JqC
Certainly not perfect, but for an additional 15 seconds of effort, far better.