Having used Credit Karma in the past, you should know that it can be...dangerously confusing...for anything other than the happy path.

Things like backdoor Roth contributions (which aren't that exotic, tbh) were very confusing in credit karma, to the point where I actually did my taxes with another service just to verify that the calculations were correct, then submitted with CK. I found several mistakes in the CK results that were due to bad/confusing UI.

You likely won't hit this problem if you're just doing a 1040EZ or 1040 with standard deduction, no stock sales, etc. But if you have any complications...sometimes there's a reason to pay for software.