If you give it correct inputs, you'll get accurate results. The trick is providing the correct inputs. What I did for 2020 is use 2019 TurboTax to prepare a simulated 2020 tax return, using overrides to correct for any errors due to values that are subject to cost-of-living changes and for any areas that are affected by changes to the tax code. If you've done everything correctly, the result on this tax return will only be a bit off because TurboTax will be using 2019 tax tables. You can then use the What-If Worksheet to copy column 1 to column 2 with column 2 set to use 2020 tax tables and you should get an accurate result for your 2020 tax liability.
I do this every year and the only errors in my tax-liability estimate that I see are due to incorrectly estimating particular items of income such as the amount of dividends that are qualified dividends and certain amounts that I don't know accurately until I receive several Schedules K-1 in March.
