I know I'm too late, but...I was also interested in this problem and found the following in the qt source code:

The difference between a GeoMapObject and a MapItem is twofold. First, GeoMapObject are always backed
by a plugin-specific implementation and do not come with a default implementation. If a plugin does
not support a specific GeoMapObject type, adding such a GeoMapObject will have no effect.
Second, GeoMapObject are not QQuickItems, thus being a much more lightweight way to add content to
a map.
GeoMapObject support is plugin-dependent, and is documented per plugin.