I've only had thee images accepted for assigments. My (limited) observation is that acceptance into an assignment (with the corresponding level 5 status) is the kiss of death for images that are simply 'ok'. That is, you'll make more money if you don't enter the assignment. Excellent images will make more money in an assignment.
family elders and seniors winners
agriculture traditional or modern winners
new beginnings winners
Take the above assignment images, for example. The two wheelbarrow images (just 'ok') have managed 3 sales each in their 23 and 13 month respective sales periods. All subscriptions. Not a good return, really! The garden image had already made 3 sales as a level 1 image before being submitted for the assignment. My suspicion is that it would have continued to sell at the lower level, but they're not worth the level 5 price.
In contrast, dandelion seeds flying into the wind has been online 9 months, with a total of 25 sales, 17 of them subscriptions. That's a good return! It's got a strong concept and probably would have sold well as a low level image, but the level 5 status means that it features high in the search results.
Taking the assignment the dandelion was in, new beginnings winners, you can see that sales aren't very high for most of the images. In 9 months, only 3 images have more than 20 downloads. Another 3 have more than 10 downloads. Another 5 have more than 5 downloads, and the rest have less than that, with 10 having no downloads at all. That suggests that having an image accepted to an assignment simply doesn't guarantee sales.
The images with more than 10 downloads are shown below. They're all either conceptually or artistically strong. They've probably made more money and had more downloads as a result of being in the assignment.
So, if your image is 'ok', leave it out of the assignment. You'll probably make more money. If you can make a great image - excellent, enjoy the sales! Of course, if you can make every image you submit an excellent image, so much the better1