The best platforms for this sort of research in the US are internal proprietary toolkits of health insurance companies, auditing companies, and law firms. The profit motives of the US health care system don't currently incentivize putting these platforms to use for science, they are mostly used for (re-) billing.
(Source: Worked for an auditing/law firm company deep on their platform once. Have friends still in health insurance/auditing/law firms.)
As for an anecdotal positive experience, one of my friends working for a health insurance company did find a correlation in lack of remissions of a very specific form of cancer and the use of a prescription drug by datamining health insurance records and helped publish that as a cure for that very specific cancer.
But given the way the industries are incentivized and fight among each other and fail to cooperate, I fear I have plenty more negative anecdotes than positive ones of things researched on top of these walled garden platforms.
The US Department of Veterans Affairs has extensive longitudinal data on tens of thousands of Parkinson's patients. They do research as well as treatment.
Sorry, I don't know the details (this was essentially picked up in a party conversation) and I'm afraid to ask because I'm assuming it was work done under an NDA given the way the project had been described to me. I don't know exactly how proprietary things were and if it would be socially/legally appropriate to ask at this point because that was under that friend's previous employment. Once again, the binds of our for-profit health system incentivizing against public disclosures and (data) scientists getting public credit for their work.
(Source: Worked for an auditing/law firm company deep on their platform once. Have friends still in health insurance/auditing/law firms.)
As for an anecdotal positive experience, one of my friends working for a health insurance company did find a correlation in lack of remissions of a very specific form of cancer and the use of a prescription drug by datamining health insurance records and helped publish that as a cure for that very specific cancer.
But given the way the industries are incentivized and fight among each other and fail to cooperate, I fear I have plenty more negative anecdotes than positive ones of things researched on top of these walled garden platforms.