Source: Department of Health and Human Services.

It's not the average middle-class worker or self-employed businessperson that Obamacare has helped gain insurance since its implementation. It's the young and lower-income minorities who have seen the biggest benefit of healthcare reform, and that's presenting a unique set of challenges for insurance companies participating in the Obamacare exchanges.

The numbers
According to the latest Gallup/Healthways survey, the rate of uninsured Americans has fallen from 18% in 2013 to 11.9% exiting 2015. The drop in the number of people without insurance is driven primarily by Obamacare and provisions that led to the expansion of Medicaid in 33 states.

Based on responses from 43,000 interviews, a Gallup/Healthways survey found that the uninsured rate among people 18 to 25 has fallen 7.6% since the end of 2013. That drop-off is in large part to Obamacare's allowing children to remain covered by their parents' healthcare insurance until they reach 26.

The survey also found that the uninsured rate fell 7.3% among those age 26 to 34. These young "invincibles" have tended to shun insurance in the past because they're otherwise healthy and insurance is costly. That they're signing up for insurance now is directly a result of rising awareness of the financial impact of unexpected illness and injury and IRS penalties tied to skipping out on coverage.

Additionally, the survey finds that 9.1% of Americans get their insurance coverage via Medicaid, up from 6.9% in the fourth quarter of 2013. That uptick stems from an expansion of coverage to up to 100% of the federal poverty level (FPL) in many states.

Digging deeper into the numbers shows that healthcare insurance reform has made the biggest impact on minorities and low-income Americans.

The uninsured rate among white Americans has fallen by less than the national average, but the rate among Hispanics and blacks has fallen by a far larger percentage. Exiting the fourth quarter, the uninsured rate among Hispanics and blacks has dropped by 7.8% and 7.4%, respectively.

As expected, high-income earners haven't seen much of a decline in the uninsured rate since 2013. The rate among those earning more than $36,000 has dipped only about 2.5%, and nearly 97% of those earning more than $90,000 are insured.

Conversely, the rate among those earning less than $36,000 has fallen 8.8%, yet 21.9% of Americans whose income is below this amount remain without insurance.

Why insurers are struggling
The findings help explain challenges facing health insurers as they attempt to profit from selling plans in the Obamacare marketplaces.

Although insurance companies, including the nation's two largest players, UnitedHealth Group (NYSE:UNH) and Anthem (NYSE:ANTM), have signed on millions of new members, these companies continue to struggle to get those who sign up to continuously pay their premiums every month. Instead, it's often those who are sickest and require the most costly care that are most diligent in paying for their coverage.

This dynamic has led some co-operative healthcare insurers to go under in the past year, and it's caused UnitedHealth Group to hint that it may exit the exchanges in 2017 because it's losing money. Obviously, if insurers fold and a major insurer such as UnitedHealth Group can't properly model pricing, then insurance premiums for policies sold through Obamacare could surge higher. Premiums for plans sold on the federal health exchange for 2016 increased by an average of 7.5%.

The picture is healthier for insurers that are running state Medicaid programs because it's the state, rather than individuals, who pay for coverage. For example, UnitedHealth Group reports that the number of people enrolled in its Medicaid programs grew 8% year over year in Q3, and that led revenue in its community and state segment to grow 21% to $7.4 billion.Meanwhile, Medicaid enrollment at Anthem grew by 786,000 people in the year ending September, leading to a 120% jump in its operating gain in Q3.

Looking forward
Insurers will have to figure out how to price plans to maximize enrollment and contain costs to keep margin in check. Until then, the bulk of their efforts should be on identifying new healthy members who need insurance and helping the healthiest of members who have signed up through the exchanges do better at staying current with premium payments. To do that, insurers will need to ramp up efforts in markets where the uninsured rate remains high and institute services that can help facilitate payments. If they're unable to accomplish that, then the biggest risk to Obamacare may not be from Washington, but from insurers' corporate headquarters.  

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.