It's not only the instructor, but also the group. It's likely that not all of the members of the group have the same objective. There are many reasons for taking a or several lessons, and not all of them are to get better. The fitness of those in the group also matters, as does the age.
At the advanced level, many students have fundamental movement patterns that have elements of a dead-end in them such as upper or whole body rotation, fore/aft balance problems, inclination without angulation, etc. If the objective is to advance to an expert level of skiing, changing those movement patterns may require lots of work that the student may find boring. But the problem of nixing the old movement patterns with those that are more efficient and result in greater ski performance takes lots of practice to move through the cognitive and associative phases to the autonomous stage. The path to improvement is likely quite individual and, frankly, will be best addressed by private lessons with a personal coach. Not cheap, and for someone who has deeply ingrained movement patterns, not quick either. I say this as someone who has been rebuilding his skiing and it has taken ten years, with most of the effort coming over the past 3. As a result of a lot of work (100 days per year over the past 5), I'm now close to passing the Level 3 skiing standard (or so my coach -- a 2 time demo team member -- and several examiners tell me).
The number of folk who really are looking to put that kind of work or time into their skiing is pretty small. If you find yourself in that camp, then you should definitely follow
@LiquidFeet's advice and find a coach -- interview them, take an initial lesson and see about their compatibility, and discuss what the plan might be.
On the other hand, if what you want, and have time for, is some quick hits, then group lessons can be a good option. As others have said, the quality of the instructor matters. The lesson club product at the Vail Resorts (at least Breckenridge and Keystone) are a great way to get a coach in a group setting to meet this objective. Otherwise, its a bit of roulette -- depending on who is assigned to your group.
Just realize that what you'll get out of the group lesson is less personalized and less of a long-term development option than what will come from private coaching.
Mike