use credit karma, but double-check using turbotax
the "free file" limitations vary widely from one provider to another. just pick one of the tax filing websites that is free for your income level (e.g. credit karma for 2017). if you're concerned about the accuracy of the provider, simply complete your taxes in a name brand provider like turbotax (but don't pay to file) then compare the final numbers to the free software.
personally, i complete my taxes in 2-3 different providers every year and compare the results. so far they have always matched eventually, but sometimes only after reconciliation. it gives me some peace peace-of-mind to know both that the software is working correctly, and that i entered everything correctly. sometimes it reveals the shortcomings of a given app. for example, this year i used turbotax and credit karma. i discovered that credit karma doesn't handle 1099-r forms correctly if they have the "taxable amount not determined" box checked (but they clearly call this out on the 1099-r screen). to get around the problem, you have to update the "taxable amount" from your 1099-r yourself, which means it might not match what the 1099-r provider sent to the irs, which might be an audit flag. turbotax does handle that scenario correctly, but only after you enter in your non-deductible ira contributions at a later point in the wizard. that means for a few minutes, it shows a smaller refund amount than it should in the running total. not a big deal, but more confusing than it would have been if the non-deductible contribution came first in the wizard.