So cover/feature images are used in a few places. My current theme uses a cover image in a tall format for featured posts, and a wide format for non-featured posts. The theme’s actual cover photos for an article can be displayed in any format. This is mostly true for my theme on mobile or desktop. And while they’re not the exact same aspect ratio, some photos definitely look better in vertical formats, and others definitely look better in horizontal formats, even when cropped a bit to fit a particular aspect ratio.
Would it be worthwhile to have an option to specify a tall format feature image and a wide format feature image so that your theme can pull whichever one is needed?