I'm with
@Calfan on enjoying the smaller group sizes. Our groups on the ABD Rhine River Cruise were just so big it wasn't really enjoyable. Some of the tours we had 45-50 people and it was seemed like organized chaos for the guides. At the zip lining for example, there was one outhouse for the entire group to use. Speaking of the same excursion, we were divided into 3 or 4 groups for the zip lining. When the last group came back there were no snacks left -- and it was over a one hour bus ride back to the boat (I was famished when my group returned from the course). Not to mention the guides seemed very scattered most of the time. I will also mention the canoe disaster in France where they put around 30 or so canoes on the river at the same time, some with experienced paddlers and others with no experience. Perhaps it was because we were the first cruise, but it is not an experience I care to relive -- ever. There was also the bicycling trip through the vineyards in Germany (?) where one of the tires on Betti's bike blew, which left one ABD guide with a group of around 40 people -- some of whom just couldn't keep up. So we spent an enormous amount of time just standing with our bikes and waiting for the those bringing up the rear to catch up, when we really just wanted to keep going (those in the rear were so winded they wanted a break and we had already been standing waiting for several minutes -- it was a bit of a disaster). Of course this is just my opinion.
People that hadn't done an ABD previously seemed to enjoy it. But we met
a lot of people that were showing off the swag they got for "complaining" to the guides (I never asked what they complained about, nor did I complain to a guide).
Since we have traveled with private guides and with ABD with groups that were just WAY too big, I know that my preference is definitely with private travel or much smaller groups. However I know that both of my kids like group travel so of course I let them chose (to some extent).