But how the plans are designed to share the costs with patients for the drugs "vary widely," the analysis found.
"We definitely see plans on the marketplace that use co-insurance for these specialty drugs," Skopec said.
This means patients taking such drugs "are seeing co-insurance rates of up to 50 percent, and that's on their first visit to the pharmacy," she said.
And even with the cap on out-of-pocket costs, those kind of cost-sharing rates can put a strain on people's wallets.
Given that, the analysis said, "It is critical for patients to be able to access and easily compare coverage and cost-sharing designs."
"We found, however, that cancer patients would face a difficult, and in some cases impossible, task in making apples-to-apples comparison of health plans based on drug coverage," Sloan and Skopec wrote.
That's because, among other things, there is no consistency in how plans provide direct links to their drug formularies. Some plans may even lack a comprehensive list of the covered drugs, and the formularies are not organized in the same way, the analysis found.
"The transparency issue is significant, because a person with cancer needs to be able to know that the drugs that they're taking are on their formulary," Sloan said.
While Obamacare plans cover more drugs for more people than in the past, not everything is covered. If drugs aren't included in a formulary, but you need them, "then you're on your own" in terms of paying for the medication, said Kaiser's Pollitz.
The ACA does establish a external review process in which a patient who claims the drugs are medically necessary can argue they should be covered by insurance. But there is no guarantee they would win, or that they would not first incur steep bills first.
—By CNBC's Dan Mangan.