We implemented relatively simple income tax reform a few decade ago in small EU jurisdiction. But law makers were very vague in some important details, and there were about 8 different interpretations. We had to implement, run and support all 8 versions for couple of years, until they decided on final interpretation. Non compliance would be fine of couple of million euro.

And that was tiny country of 5 million people with clean newly written laws. Not babylon with 300 years of baggage like US.

That's true but TBH, even those that are wildly successful aren't actually that great. I've implemented Workday as a replacement for the user-facing portions of SAP's HR and while it looks good, it's nowhere near as flexible. There's a lot of stuff I would expect to be available in Workday and just isn't. That surprised me as Workday is pretty much the HR market leader.

TLDR; Opportunities are there for new companies to do ERP better. It's not easy though...