I have little European experience, and our first European cruise is 2 months away. But something to consider is your group size. It surprised me how hard it was to find nice accommodations in Rome for a family of four, that didn't require two rooms or an expensive suite. My understanding is that much of Europe is like that, where the average room sleeps 2. When you factor in two or more rooms compared to one room on the ship (depending on your party's size and comfort sharing a room), plus transportation, plus food, plus evening entertainment, a cruise can be pretty economical. Disney, maybe not as much, but still not terrible, per day, compared to our time in Rome prior to the cruise. But, we travel pretty well, preferring non-budget hotels, restaurants, and transportation. So it may also depend on your travel style.
We picked the Italy/Greece cruise over a land trip because we want to see several of the Greek islands it goes to and I didn't love the idea of catching ferries, checking in an out of hotels for two weeks, and having no down time because we feel compelled to go go go. With the cruise, we get to see Sicily, which isn't too common for U.S. to Italy travelers, the Amalfi coast, which is also high on our list, and have two sea days where we are forced to stop and relax. Plus, following Rome, we unpack once and have evenings with pre-set meals and entertainment. It just fits our energy levels and personalities too. I know the ports will be packed and we will miss the immersive experience after Rome, but the benefits outweigh that for us, at least for this itinerary.