For instance, health plans aren't supposed to require prior authorization for a psychiatrist visit unless they also do so for a "comparable" physical health service. The problem, of course, is how the word "comparable" is defined and interpreted.
"Even if you're not trying to exploit the ambiguities but are trying to do a good job, you still might get into some controversial situations where folks might disagree, and disagree on a legit basis," Frank said.
And, both Frank and Glied noted, there are cases in which insurance plans might try to take advantage of those ambiguities.
Take, for instance, the question of "medical necessity." Insurance plans can argue that a mental health service isn't required for someone's well-being. Advocates argue that, even in the wake of the parity laws, that argument is used as an excuse to deny care, and that it is used more often for psychiatric cases than for, say, treating someone with diabetes or a hip injury.
These barriers more often affect people with less severe mental health conditions, but who still require care, Glied said.
States have amped up their enforcement, in part due to federal support, Volk said. But they often rely on reports from consumers who have experienced parity violations, which assumes people will realize their plan has broken the law.
And issues exist beyond parity. Research suggests that insurance networks still fall short when it comes to including an adequate or even robust panel of behavioral health providers who accept their coverage.
That's in part because of distinctions in mental health care -- whether someone specializes in cognitive behavioral therapy or talk therapy, for instance, or a psychiatrist's age or gender or life experiences. Plus, many behavioral health specialists simply don't take insurance.
Those issues, Frank said, come in part because of the distinct complexity of mental health coverage.
"Mental health is not just one thing -- it's a whole bunch of illnesses. You want people to get the care they need," Frank said. That means that devising effective policy "is a balancing act."