The ACA is still a terrible law despite its good intentions. Lets start from the beginning: The US health system itself is completely dysfunctional (they pay more money for worse coverage), because it is the amalgamation of various different systems. A very simplistic way of looking at the US system is to say that it is a mix of the German, Swiss, Canadian and UK model with a mix of randomness added. Often it takes the worst aspects of each system. The result is something completely incoherent, that creates a shitload of problems. One (of many) issues is the problem with pre-existing conditions. It is a very real problem, that needs to be fixed. Still the ACA is extremely flawed.
First of all it tries to hide the costs of the health-care by putting the burden not on all tax-payers, but just on few sub-group of them (e.g. young, healthy people and some people, whose premiums are skyrocketing). There is no moral or functional argument for that; that is just down to politicians trying to hide the real cost of this legislation. This mechanism ("individual mandate") doesn't work very well. Too many healthy people refuse to sign up, which means that the costs don't balance out. That means that even more hidden government subsidies have to be paid.
Additionally the government has to define what health-care plans actually have to include, which opens up another can of worms.
But the real problem is that it creates strong market incentives to offer bad health-care for people in need. Insurance companies incur a loss with people, who have pre-existing conditions, but they have to cover them. So it is not surprising that they try to lower the cost by reducing the quality of the treatment for these people. If one company attracts too many people with (expansive) pre-existing condition, they'd go broke. So the ACA institutionalizes a vicious circle, that puts down-ward pressure on the quality of coverage for the most vulnerable people.
It also leads to insurance companies dropping out of this market, which leads to less competition. Many areas have already just one insurer left in this market. It goes without saying that this is not helpful at all.
tl,dr: The ACA tries to tackle a real problem, but it isn't a sustainable solution. It also doesn't address all the other major issues of the US-system. It is terribly crafted legislation. Obviously the Republicans have no solution to any of these things either. Historically they never had any real interest in health-care politics at all. Only once Obama made it "his thing", they started to cry foul, because they are partisan idiots. Any useful reform of the US health-care system needs to create coherence, not additional patch-work.