It's a major annoyance that group items can only be added at the end of the list.
It's not so bad when the group items only have one field, since dragging them into place isn't a major pain, but if there are lots of fields and so each item takes up a good amount of space on the screen, and especially if the list is already quite long, dragging it into place is very painful. If entering several items all at once which need to be somewhere near the top of the list, it's a very poor experience.
Slices can be inserted at arbitrary locations, so why not group items too?