Just my personal experience:
In 40 years as a health care provider, having seen 1000s of patients, I've never known anyone with a chronic illness cured solely by prayer, or some similar kind of religious intervention. I've seen plenty of people improve significantly who prayed and received medical treatment. I've seen some people with diabetes and hypertension eliminate their need for medication. Many of whom probably prayed. But they also followed a diet, exercised, and lost weight. (Prayer might have helped them do this, and if so, that's great. But if they lost weight without praying, they'd almost certainly have the same good outcome.) I remember one patient quite well. She and her husband were both quite religious and often attended evangelistic crusades that would come into town. She had quite advanced hip arthritis. She was in a lot of pain and needed a cane, and sometimes a walker. But she was very afraid of getting hip replacement surgery. One day she comes into the office in a totally upbeat mood. She'd been to a healing ministry event a few days earlier, and she now felt great. The minister laid hands on her, and he and the whole congregation prayed for her deliverance from disability. She immediately felt the pain leaving her body. She said she walked through the arena without pain for the first time in years. She believed that she'd been cured. I recall that she really did look much better and was walking without assistance. So I did an x-ray of her hip. She may have felt better, but she wasn't cured. Her hip joint was the same--bone on bone, no joint space (meaning no cartilage,) a partially collapsed femoral head, and large bone spurs. I told her her x-ray hadn't changed, but it was great that she was feeling better. And I asked her to keep in touch. And a week or so later she was back in the office. The pain had returned--and maybe worse than before. (I think she might have been in a wheelchair.) But now she was also terribly depressed thinking that she didn't have enough faith, or that God didn't love her, or she had committed some sin that caused her relapse. So this "healing" ministry not only didn't work, it provoked a crisis of faith, which made her feel worse.
If there is a hell, there ought to be an especially scorching place for these charlatans.