Essentially, the moment a customer becomes more trouble than they are worth, they are dropped. This is true with other types of industries, but if your health insurance drops you when you get cancer, you can't get more health insurance, and you die.
Same with house insurance, car insurance, ect...
And it causes death or financial disaster all too often.
Some would argue that "this doesn't happen" or "it's illegal".
1) It happens ALL THE TIME.
2) It's illegal, but if you don't have the means or education to fight it. You are pretty much done.
It's the fundamental nature of insurance companies to milk healthy customers while dropping unhealthy customers. It's just too tempting and they are too protected by our legal system for them to not do it.
I know this is pessimistic, but as long as you realize this fundamental imbalance in the relationship with you and your insurance companies, you can mitigate it to a certain extent.
But, really, the only way to completely mitigate it is to be so rich that you don't even need insurance.