FreeTaxUSA made me manually input my stock transactions last year, via a paper PDF I had to print and mail separately. For most situations it is probably fine but its investments support seemed limited.
Why would an open source solution improve on that though? If anything, that would be way lower on their priority list as anyone doing large numbers of investments would (should?) have money to pay for a nicer UX solution and probably would want to, as the risk of an audit is a bigger 'cost' than $50-80 in filing fees.
The sheer number of changes that the Tax code makes every year would be hard enough to keep up with for an open source project, let alone making integrations with all of the brokerages to make this process less painful.