When you have a contract with an insurance company, that service provider has to pay up or you can sue them if they don't. Sure, there might be a co-pay or a high deductible, but if you draw an unlucky card with cancer / heart / cardio disorders, they will fork out six-seven figure sums to get you better.
"Christian Healthcare Ministries" and the like have no obligation to pay out and you are gambling on their goodwill, which can be revoked at any moment with no recourse. Further, they often have _lifetime_ caps of ~$100k per diagnosis/condition, and in case of any serious illness this will be chewed up in 3-4 months, then you are on your own.