a) There is no such thing as a guarantee that your luggage will not be missing, even after a non-stop flight. Moreover, this is independent of the airline flown and the airport you board.

b) Neither TSA pre-check nor Global Entry will have any effect on potential immigration lines in Italy (or anywhere else for that matter), except for coming home to the US (Global entry).

If your stuff is small enough to be accepted as cabin luggage, not checking it reduces the chances of being lost bur hurts your... shoulder!