I wasn't a heavy user of the Canadian health care system (Ontario), but there are some minor things I like better about the US system.
In the US, I can book a specialist appointment without a referral. This gives me the flexibility to get second opinions easily, or even a first opinion without wasting a bunch of time. And the technology tends to be a little fancier in American health facilities.
Why does it matter? In Ontario, I didn't have a GP. As others have mentioned, it could be hard to get one and even if you did have one, it wasn't always easy to get an appointment. I used a local walk in clinic, instead. This worked out fine until one incident - I was very ill and the doctor at the walk in clinic said that the reason I was sick was because I was allergic to my cats. He said this without an allergy test or any history of pet allergies. When I argued with him, he told me "Who's the doctor here?! Get rid of your cats or you won't get better." I don't know why he hated cats so badly. He refused to refer me to an allergist to get tested for this hypothetical cat allergy, and since this was in Canada, I did not have the ability to make my own allergist appointment because that's a specialist.
My brother's GP was in the same building, but not accepting new patients, so I walked up there and begged them to let me on their roster and get me an appointment. They did (I might have been crying), but I had to wait a week. In the meantime, I got more and more sick. In the end, it turned out that I had mono and a case of the flu on top of that. But enough time had passed that it had basically run its course and the doctor said there wasn't much he could do for me.
Another anecdote: My brother has severe bipolar disorder. He's used mental health services in Ontario, BC, and Alberta. He says that each province is different and the level of care varies considerably. I can't remember which one was best and which one was worst, though. He was in major cities in each province, too (Ottawa, Vancouver, Edmonton). For all of that, he has had access to services in each province and inpatient treatment when needed - however, good outpatient care keeps him from needing inpatient care. The Canadian healthcare system has created a safety net for him that I don't think really exists to the same extent in the US.
When he was first diagnosed, he had a psychotic episode. The police were called. He claimed that he had weapons (he didn't - he was delusional - but they didn't know that). They dealt with him compassionately, helped calm him down, got him to the hospital, where he was immediately admitted and stayed for over a month while he recovered. I don't really have confidence that the system would have worked so smoothly for him if he were in the US when that had happened.
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I wish the US government would negotiate drug costs on our behalf, and I wish they would regulate hospital costs better. As a user of healthcare in the US, it can be incredibly difficult to find out what something is going to cost you. There is no way to be an informed consumer of healthcare, because you have no way to "shop around." In that kind of consumer environment, we rely on the government to protect us. But the US government fails utterly to do so.
I think ACA has made a lot of strides with pre-existing condition and not letting insurance put a cap on care. Pre-ACA, my friend had some health concerns, and after visiting a specialist, it seemed that he might have Multiple Sclerosis (relapsing remitting). The specialist told him that since the symptoms weren't bad and since this was a first occurrence, it might be better NOT to get a diagnosis. It could be years before there was a re-occurence and there might never be one - but if my friend had a "pre-existing condition," he would have a very difficult time getting insurance, possibly for the rest of his life. In the meantime, the treatments for MS wouldn't necessarily be helpful to him until the MS progressed. He chose not to pursue a diagnosis or treatment.
On the other hand, now that insurance providers can't charge you extra for pre-existing conditions, costs under ACA have escalated dramatically. They say that people in the US are sicker than anyone predicted! Go figure. I think this strongly illustrates why our government needs to step in and regulate costs. Health insurance companies might not be making any money under ACA, but drug companies and healthcare providers/hospitals certainly are.
I'm still sometimes unsure, if I was diagnosed with a serious illness, such as cancer, whether I would stay in the US for treatment or go to Canada. I'm still a little afraid that there could be some loophole in my insurance that could cost me hundreds of thousands of dollars. I still don't "trust" the US healthcare system.