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Enrollment in Medicare Advantage plans has grown substantially in the past few decades, enticing more than half of eligible people, primarily those 65 or older, with low premium costs and perks like dental and vision insurance. And as the private plans' share of the Medicare patient pie has ballooned to 30.8 million people, so too have concerns about the insurers' aggressive sales tactics and misleading coverage claims.

Enrollees, like Timmins, who sign on when they are healthy can find themselves trapped as they grow older and sicker.

"It's one of those things that people might like them on the front end because of their low to zero premiums and if they are getting a couple of these extra benefits — the vision, dental, that kind of thing," says Christine Huberty, a lead benefit specialist supervising attorney for the Greater Wisconsin Agency on Aging Resources.

"But it's when they actually need to use it for these bigger issues," Huberty says, "that's when people realize, 'Oh no, this isn't going to help me at all.'"

Medicare pays private insurers a fixed amount per Medicare Advantage enrollee and in many cases also pays out bonuses, which the insurers can use to provide supplemental benefits. Huberty says those extra benefits work as an incentive to "get people to join the plan" but that the plans then "restrict the access to so many services and coverage for the bigger stuff."