Absolutely incorrect.
Our current tax system with it myriad credits and deductions does that. MORE types of taxes does not add any such opportunity to "tinker" with rates.
You can do the exact same think you complain about in our current code. NOTHING is there to stop the Political Class from doing that tinkering right now.
So again your critic is without merit.
9-9-9, in fact, seriously reduces the political classes ability to tinker since it is transparent and everyone pays. There is no ability to pit one political group against another by demagoguery like "making the rich pay their fair share" or to write breaks into the code for fat cat lobbys
There is a much greater opportunity to tinker with the current tax system with it mind numbing complexity and opaqueness then there would be in the relatively transparency of a 9-9-9 system