Earlier quoted context omitted.
Open source can solve this problem. Any service you choose might be missing your edge case. But if you can see all the code and figure out where to plug in your edge case, then you can contribute it for all users in the future to enjoy. We've already seen people show up and implement features we hadn't researched yet. I encourage you to check it out.
The consequences of botching it are high, so taking commits from external contributors requires detailed and time consuming reviews. Further, people don't know what the edge cases actually are at scale. If I go do my taxes and it doesn't account for some weird thing, I simply don't know that this weird thing even exists in the first place to justify a PR to the tax service.
You might not know the edge case exists, but out of all the people that have that edge case hopefully there is one that does know about it and can implement the fix.
Also I'd be curious to know what edge cases you're thinking about. Many people just have a W-2 and a few 1099s. If someone has a complex business or some other concern they're probably at least partially aware.