Illustrator Vector Image - Quality Issues

I’ve had an image rejected for quality issues and could use some help seeing what I’m missing in my art or technical setup. The list below has the items I’ve checked. 

• It does not contain raster elements
• I don’t use JPEG compression in the preview nor do I export to a JPEG
• I have organized the image in layers so it can be customized
• I didn’t outline the fonts, but that hasn’t been noted as a problem before
• I target 8K monitors so it is 7680x4320. I’ve had hundreds of images at this resolution accepted so I don’t think this it. Maybe they are watching this more closely?
• File is less than 1 MB in size.
• I do use swatches to designate cross-sections. Maybe I should expand these so they aren’t swatches?

I’m a PhD student studying engineering and not a professional illustration. Maybe this is an aesthetic train wreck and I can’t see it. Should I rethink the artistic styling and colors?