First and foremost, integrity and honesty of the dentist should be the first requirements. You can never tell how technically superior dentist is. But you need to trust the dentist for long period of time. Big, complex, and difficult case often takes more than a year or even two years. When patients questions the intentions of my recommendations, sadly, I know that the patient is not ready to move forward to treat the work they deserve.
Secondly, the big case has to be planned carefully. Most of the patients think that the big, complex, and diffcult cases fail at the execution or technique stage, but reality is that it fails at the planning stage. As Benjamin Franklin said, "failure to plan is to plan to fail." Most of the true big cases are started and finished in less than 3 % of cases out of 100%. In other words, it is extremly rare to get treament done. I know, it is amazingly low number. But that is the reality.
Third, ask dentist what the final picture would look like at the end. It is perfectly okay to ask the question from you. Different dentist use different tool to visualize the end product for the patient, but in one way or the other, through simulation, mock up, or similar case photos, dentist can definitely show you what is going to be look. Dentist must have mental image of finished work. In comparison, often patients are clusless what it is going to be look like.
Big, complex, and difficult dental work is not something all dentist aspires to treat or deal with. It cause headache to even some dentist! Not all dentist should touch/enjoy the process. But, to me, the reward is much more satisfying than routine dental fillings. So, it is perfectly fine to ask another dentist or get second opinion than your previous dentist. By all means,it is not that your denist is not good. Good dentist is a dentist who knows and abide by the boundaries. I would rather choose a dentist who competent in small expertise than not so cometent in large expertise.
If someone told me at my dental school graudation that I would be the dentist who treat those big cases in New York, I would laugh so hard. But life is funny. . . Now, I have a burning passion to treat this small but rewarding segment of dental population.