I don't think anyone is making the claim that UnitedHealthcare as a company is in trouble financially. They are pulling out of the ACA because it is an unwise financial decision. They are losing money and not even breaking even in that controlled marketplace.
Why must a company engage in unwise practices (losing markets) for them to be credible? At the core, insurance is a business designed to make money and help people. They can't help people if they don't make money. But the dual purposes of making money and helping people are not contradictory.
Perhaps, just perhaps better management would allow them to make money. Isn't $66 million in one year for the CEO a bit obscene?