Everybody that I've led through a CoveredCA application stumbles when they get to the subsidy section. The problem lies in the estimation of next year's income. The law asks you to predict the future and that's not possible to any degree of accuracy. California addresses the issue by requiring enrollees to login and update their information every time their income changes...a requirement that'll be forgotten (or ignored) by most people. I assume other states have a similar requirement.
My suggestion...Change the law to base next year's subsidy on the current year's income. If someone is signing up in the October through December enrollment period, they should be able to estimate year end income far more accurately than next year's income. And if you're signing up after January 1, the books are already closed. A (not-so) simple change will eliminate a lot of surprises when tax returns are completed and people find out they under/over estimated their subsidy.