Its not incompatibility...things like withholding are not available for Suitetax implementations meaning you have to figure out how to handle it yourself. Lots of global entities have to handle withholding and just considering the possibility of reengineering your processes is daunting,. If you are a new customer you dont have any ties to the old setup and SuiteTax is certainly an improvement. Too bad Oracle isnt pushing it harder after all of their investment