Why won't a single miniloader transfer directly between two inventories?

By definition, a loader is something that removes items from an inventory and puts it on a belt, or vice versa. Thus to move items between two inventories, you need two loaders. Miniloaders are 1x1 footprint, but they are still loaders.

Just how UPS-friendly is "UPS-friendly?"

Testing by Optera shows that miniloaders can be substantially faster than normal loaders (e.g. Loaders Redux, Deadlock's Loaders, Krastorio built-in loaders, etc.) in some circumstances. Actual results will vary based on how fast your belts are (very fast belts require multiple inserters to keep up with them), and how often your belts back up.

Miniloaders are always faster than any other loaders when interacting with vehicles, such as cargo wagons.