I had one procedure vanish into the waiting-for-approval depths without ever resurfacing, and one expensive test that my insurance company took six months to pay. I haven't had similar issues lately; lately I "just" have hundreds of dollars in deductibles.
I had great insurance thru my company. United Healthcare. I had low co-pays, I didn't need pre-approval for anything, I could set aside money tax free to be used for anything unpaid by insurance. They would pay for almost everything, just not always what the doctor charge them. I would pay the difference. That is why going out on my own freaks me out! I know I have been spoiled..