Currently, there is no way to add metadata to an image like 9-slicing information, so any image used for an arbitrarily-sized classic UI button becomes stretched and warped, even using image type="Sliced"
This makes robust-looking UIs extremely difficult to create.
Pleae allow Unity Sprites to be imported into the UI asset manager, as materials can be for other things.
Sprite importing seems like the easiest to implement, but an XML attribute like sliceMargins="5 5 10 5" to define slicing at the element level would also address the issue.