Open enrollment in Obamacare starts Sunday, in what appears to be a pivotal time for the controversial healthcare law.

Obamacare started on a rocky road in 2013 with the botched rollout of healthcare.gov. The website has since improved and added new features, and about 10 million people have paid for Obamacare on the insurance exchanges.

But challenges remain for the marketplaces to remain viable, a key indicator for the healthcare law. Here are four key challenges facing them:

1. Keeping the Obamacare marketplaces competitive

As of Oct. 30, 11 taxpayer-funded Obamacare startups have shut down due to financial problems.

The nonprofit insurance cooperatives were created to provide more competition for Obamacare marketplaces. But the Obama administration has said that not all of the co-ops were destined to survive since they are startups.

There is also the question of whether regular insurers will remain in the marketplace. Robust competition is a key way to keep premiums down for enrollees.

Eleven states have announced that one or more carriers will exit Obamacare markets. However, at least 14 states have one or more new insurers applying to participate in the exchange market, according to the healthcare research firm Avalere Health.

Insurance giant Aetna announced Thursday it would stop selling Obamacare plans in two states and Washington, although it added Kentucky for 2016.

The reason is because Aetna wanted a good cost structure for an affordable product, Forbes noted.

"Carriers continue to see value in investing in the exchange market," said Aetna CEO Dan Mendelson. "In 2016, consumers will once again choose among several competing insurers and between diverse benefit designs."

The number of insurers selling plans will increase this year, with five insurers per county as opposed to four last year, according to administration data released Friday.

However, consumers will have fewer plans to choose from. For 2016 open enrollment the average consumer can choose from 50 plans, down from the average 58 in 2015

2. Reaching the remaining uninsured

The administration set a pretty modest goal for Obamacare this year, saying that only 10 million will have healthcare coverage by the end of 2016. That is about the same number that pays for Obamacare now.

A majority of the 32 million remaining uninsured have low incomes, so the administration is stepping up its education on how to receive subsidies. A recent report from the left-leaning think tank Urban Institute suggested that tax preparers such as H&R Block could help people to see if they are eligible for tax credits when they file their tax returns.

A recent government report said projections are going to be hard to pin down because the marketplaces have been around for only two years.

"As marketplace coverage becomes more widespread and the size of the uninsured population eligible for enrollment in coverage through the marketplace shrinks, the remaining uninsured may be harder to reach, slowing enrollment growth," according to the report from the Department of Health and Human Services.

3. Finding more affluent enrollees

Earlier this year, the Obama administration released data that found 84 percent of the nearly 10 million Obamacare enrollees receive tax credits.

A March analysis from Avalere Health found that participation levels in Obamacare decline as incomes increase, even among those eligible for both premium subsidies and cost-sharing reductions.

The firm suggested that a greater emphasis on mandate penalties could help entice enrollment.

The law's individual mandate imposes financial penalties for those who don't get insurance, with exceptions for low-income Americans. The penalty for 2016 is a whopping $625 per person or 2.5 percent of yearly income, whichever amount is the highest.

Without finding more enrollees, then Obamacare signups could stagnate which could in turn affect the marketplaces, Avalere said.

4. Educating enrollees about out-of-pocket costs

The administration hopes to take away some of the sticker shock when enrollees receive their first medical bills.

Healthcare.gov, which residents in 38 states will use to sign up for Obamacare, now includes an out-of-pocket cost calculator. When an enrollee selects a plan, he or she can calculate how much he will pay under that plan.

That isn't an issue just for Obamacare enrollees.

More employers are switching to offering only high-deductible plans, which require workers to shoulder more healthcare costs. A recent survey of 140 large businesses found a 50 percent increase in companies offering only those plans in 2015 from 2014.