A future improvement I am looking into is adding unit tests for tax calculation. If the project gets popular enough I would probably hire an accountant to verify everything.
When using a well-known site like TurboTax, while I'm totally appalled by their business practices, I have at least an assumption that I can do my taxes half decent (and not too much more than that either). It will be really hard for an open source project to gain enough momentum for me to have the same confidence. For a lot of other open source projects I can just try, and see what happens. But I'm not going to try and see what happens if I'm ever getting audited. And I'm using (begrudgingly) TurboTax exactly because I know nothing about tax code, so the unit tests (yet to be written) would not tell me much. I find this project very sympathetic, though, but I'll have to pass.
That's absolutely fair! I think finding a solution to this will be key to any kind of mainstream adoption