SmartAsset is only a rough approximation based on standard deductions/exemptions and the rate structure. It excludes tax credits and other provisions that make taxsim much more accurate, especially for low-income households.
But again taxsim is missing local income taxes entirely. That makes it automatically inaccurate for millions of people. Smart Asset is an estimate but it's at least accurate for the cases that it covers and doesn't claim to be a complete tax calculator.
Anyway taxsim is obviously the more complete tool, but without local income taxes it's unusable for many people like me who live in a state that has local income taxes too.
