We booked this new tour the day after it was first posted, and went the following week, so all the kinks probably weren't worked out. We asked Viatour to refund the difference between the price of the group tour and the private tour, because we ended up getting the group tour. They were more than generous, and we only asked once, via email.
Even in a bi-lingual group, the tour was still worth it. You should plan to go to Santo Domingo.
If you sign up for the private tour, call at least 3 days in advance to confirm pickup time. It took us a day to figure out how to find a valid phone number for the tour operator. Hint - there are two area codes in Dominican Republic, 809 and 829. Viator provided us with a phone number whose area code was wrong.
We definitely enjoyed the tour we got, which was a bi-lingual tour of 40 people. I'm not sure whether a private tou would have improved the experience that much. It was a good tour.