Main problem with dental insurance is that it is highly subject to adverse selection, and generally comes with such high deductibles/copays that it should be looked at a dentist panel discount program. By adverse selection I mean that the buyer can choose to do many dental procedures at a time that is convenient to him. You can wait on a lot of the stuff that, while legitimate exercise of dental practice, is not immediately essential to your health. That means that a buyer can get dental insurance, then go to the dentist with a mouth full of problems he has neglected for a decade and want the insurer to pay for it all, and then has the option to drop the coverage the next year, or just stop paying the premium.
Partly because of that issue, dental insurance actually covers a small proportion of the dentists' "usual and customary fee." That fee is established by what dentists in the area say they will charge a patient who can pay an unlimited amount of cash for retail work. Many patients of course never pay that cost in cash, and dentists on average do the work for a lot less. Dental insurance will typically ask you to pay half the cost of a procedure and reimburse the dentist much less than half of that cost, so the insurance company is getting a huge discount from the dentist, and he benefits because you came to him for care that you otherwise would have foregone, and the co-pay you fork over more than pays for his time. The "free" benefits in most dental insurance plans like a free exam and x-rays once or twice a year, can typically be had for really for free without dental insurance. I get a flyer in the mail about once a month where a local dentist who is having a slow time hopes to recruit new patients by offering a free exam, x-rays and/or cleaning. Since there are a gazillion dental "problems" that you probably would not have noticed without the free exam, it is a ticket to an expensive show.
Finally, it is unusual to find dental insurance that does not have a relatively low annual benefit limit. The point of insurance after all is to have a source of payment for a catastrophic expense that you can not cover from savings. If some dental disaster occurs, that will cost tens of thousands of dollars, you will probably find your coverage quits. Often that limit is as little as 1000.00, which means after the dental insurance has paid the dentist that amount in a year, they pay nothing. Since the insurer is getting a discount, you will actually receive significantly more coverage measured in retail fees, but it is not difficult to imagine a scenario where you will get stuck.