Bruce..thanks for adding comments here. It is good to hear an opinion, and some added information from a ship's officer.
I think some of MercedMike's post comes from a viewpoint of someone with some "mobility issues". I have no doubt those people think they are much "safer" going with a cruise line excursion. And I have no doubt they feel that way because that's what the cruise lines tell them
The point of going to the same places... from many ports the "major sites" are the "major sights". BUT... with a private tour you can spend more time at those sites, plus still have time to customize your tour to add different venues.
I've been to St Petersburg, Russia several times. Each time I've seen the cruise line's tours lining up to wait for admission to sites, as we've walked right past and "front of the line it" and walked right in (all prearranged).
Most of the luxury lines are now offering to arrange private tours for guests wishing that option.
I do get it that some folks prefer the "cocoon" of a ship's tour. But I do wonder how many have just been programmed to believe that's the safest way.
My own main objection to ship's tours is that they move as fast as the SLOWEST person on the tour, and of course the mandatory stops at "recommended stores" for shopping, or their stops for a "local lunch" (that's often at a truck stop).